How do you treat an angry employee with continuous aggressive mood?

Have a private meeting with him\her.
Mention how his\her aggressiveness and angry outbursts effects you or others in your office.
Potray or give an example of how he or she behaves and ask if they would want to be treated in this manner.
Also ask if there is a personal issue that he would like to discuss.
There could be a personal problem relating to this individuals outburst and actions.
Mention if it continues and constantly effects other employees, a verbal warning will be issued and further action taken by management.
Also mention that you would not like the situation to continue as you value him\her as an employee.
This I think would boost the accused person character and may stop his\her bad behaviour.

The first time will give him a letter warning if it does not work will permanently disconnect

First of all, try to have a meeting with this employee in private.
Ask him what is the problem.
Maybe he is having a bad moment not related with the job and there is the only valve of scape for his bad manners.
In that case tell him you understand his problems but he has to change his behaviour because it's not his colleagues fault and they also deserve respect.
If it is an specific problem with somebody at work, have a meeting with the other person and try to make them understand you all are in the same boat and they don't have to be friends but they have to be good workmates.
In case this worker is rude and unpolite because is the way he is, I would advise him once, maybe twice, but at the third time maximum he would be fired.
Why? Because I understand a boss must keep a good feeling between workers and workmates and, protect the rest of the employees.
In this specific last case things can only go worse with somebody like that in a company.
And other employees could copy his behaviour.
Sometimes one has to show who is the boss.

First try to investigate the reason of anger and then it will be easy to calm him down and change his attitude to positive one.
That's what I did a number of times and got success.
The reason of anger can be either his work conflict or family conflict and family conflict sometimes emerges due to work conflict.
And work conflict arises due to either salary or work environment or wrong person on right job, When you know his/her problem then invite him/her to tea or lunch and sort it out,

If you have an overly aggressive employee and this behavior is patterned and is affecting the environment, something needs to be done immediately.
This is toxic behavior and can impact your business.
This person should be immediately counseled.
Maybe they are or are not aware of their behavior and its impact but they need to be made aware so that they can take whatever steps are necessary to rectify the behavior.
In this one on one counseling (if it is within your capabilities) you can let them know that you are there to assist but additionally inform them on what steps will be taken if the situation is not addressed.
It may help to offer a few resources.
Let the employee know that you have those resources readily available and will eagerly help with the process.
Those resources may include anger management classes.
The idea is to inform the employee (one-on-one) of the behavior and its impact.
Secondarily, let him/her know the consequences of such behaviors and the course of actions that will follow if it is not rectified.
Offer services to help in whatever way you can in curbing the behavior.
Lastly, let the employee know you will be monitoring and if possible give them a definitive time line where you will meet again to gauge progress.

First lets understand that this person was selected for that particular job role after certain procedures and screening.
Therefore this person has established his/her potential, knowledge and work related skill sets and is an asset to the organisation.
Some human beings are quite aggressive by nature and it's nothing negative.
The team leader has to treat this as a potential and control the same by deploying it in the appropriate roles for better returns and the employees job satisfaction and motivation.
As far as anger is concerned, every human has certain level of anger potential and it might trigger with some minor issues with some and with major issues with others who are more tolerant.
Certain people given authority start showing anger which is VERY ALARMING.
Such angers are expression of frustration due to lack of leading by setting examples, less of listening, lack of trust, lack of confidence in the job role, etc.
Anger related issues are generally discussed outside the office in a friendly environment.
An immediate assessment has to be done and mutually agree upon an action plan and an outcome.
One will have to work hard on this knowing that firing is the easiest way but quite an expensive way.
Please remember that hiring is not an easy exercise and needs quite an investment of time and money.
Firing should always be kept as the last resort if all possible corrective actions have been exhausted.

Try to understand his problems and solve them

You need to talk with him in four eyes, listen to his problems patiently and propose a solution if you can.
But it's not your job to solve the problem.
I mean maybe there are a lot of possible things why he feel anger.
If he angry about the job, you'll have to solve.
Long time no way I let this behavior because it can poison the atmosphere, which would have hurt the company.

The first thing to do is to have a face to face meeting.
You ask him to express himself not to worry about anything else, you just listen to him and note down on a paper his issues and problems.
After you have listened to him, you advise him on matters you think you can advised on.
try to counsel him and put infront of him several options or even solutions which might be useful for him.
Your problems and issues are your problems and issues.
if you want to be independent then you are the only one to solve your own problems.
No one else can solve your problems and issues.
You cannot change someone's behaviour but you can create barriers such as if that person continues to be agressive after the fisrt meeting then you should give him a warning letter and despite all these he continues then you should take disciplinary actions.

Such employees have to be dealt very compassionately.
We are all human beings and we all have emotions and feelings which must be respected.
In my opinion he/she needs help because my experience is no one deliberately remains angry and in aggressive mood.
There is normally a personal problem which he/she is not able to deal with.
For example when you have your spouse continuously nagging at home on a trivial matter one would remain irritated at work.
So such people tend to vent out their frustration at work place by such behavior.In my opinion their manager should try to advise their close friends to find out the reason and try to help.
For work reasons if one is angry and remains irritated in my opinion this person is overloaded with work or their is a situation.
When I say situation I mean at times an employee is functionally reporting to two or more managers who are pressurizing him / her to attend to their requirements / queries first, the example could be an accounts executive who has to release the incentives on sales and has to derive figures from departmental or product sales data himself.
He can do one at a time but every department / product marketing manager wants his team to be paid by immediately.
Another situation for such behavior , I had an opportunity to offer consultancy for performance improvement with a facility management organization where I found such type of behavior very often with maintenance technicians as they had to take the firing from residents for no fault of theirs.
I will quote one incidence,one of the resident had complained that she was not getting water and the technician found that pipe was damaged and needed replacement.
The resident requested that it should be replaced/ repaired by the the time she returns back from work in the evening.
Technician immediately ordered with his purchase department to get him the pipe by noon but it did not come, the resident on her return blasted the technician who had to take the brunt as resident was not prepared to listen, she required water and she should get as she was paying for this service.
This person I found always was in angry and in aggressive mood amongst his colleagues.
It is a very interesting question and the concerned person has to be dealt very compassionately please try help him / her if there is a personal problem or identify and rectify the systems in the company if this due to reasons at work.
In both the cases my advice is to deal with the subject compassionately , it will certainly help

I advise you investigate his problems and solve them.