What is the best way to handle users who constantly complain?

first i'll ask him or her about what they complaining about then i'll try to solve thier problems and make them comfortable by listening and trying my best to fix all what they saw uncomfortable ..but all that for the real problems only not those who just nagging ..  

Fisrtly talk to them OR RATHER Hear them out. Listening (not the same as hearing) to customers helps solve50% of all complaints.

- Find the root causes of the problems 

- Resolve what parts of the problem can be addressed immediately

- Ensure those problems do not repeat again

- Make an action plans for solutions that need a bit more time

At the fag end if you find some areas cannot be resolved due to lack of resources, you need to talk to the customer again

Explain to the customer how you value his feedback and what actions you took to resolve it. Also explain the one's that are not practical to resolve and why. And if he / she still wishes to take their business elsewhere, it's their choice. (99% of the time you will find them satisfied and coming back for more business) 

Be politeListen to their complaint/problemDon't make false promiseTreat them with respect no matter how angry they make you And wish them well

Constant complain from a customer due to a mistake from our side can be handled by listening carefully to the compliant, understating its dept and real impact to the customer and trying to make a solution for that impact rather than a constant rectification and acceptance of our mistake.

Open up for suggestion,do some reverse psychology on the situations,and tell the costumer that he/she,is on top priority and made some comparison with other costumer with similar problem and showed that on performance level of costumer satisfaction,your company had quite having an excellent performances and not to give hope because he/her complaint are under extreme scrutiny and further review.

Being customers, employees or employers, we are all people. And all we want, is to ventilate. I learned by experience that simply by letting things out is a help tool on its own. Someone's dissatisfaction is mostly on the moment; therefore, when they talk and share their concerns, they feel better after, especially when the employee ' employer shows some kind of sympathy and understanding. It is important to let the customer talk and say everything he / she has to say without interrupting. 

ANYBODY COMPLAINING CONSTANTLY IF I AM THE PERSON MANAGING HIM I WILL NOTE ALL THE COMPLAINTS , AND  ANALYSE THESE COMPLAINTS ,IF THE COMPLAINTS ARE  RELATED TO THE COMPANY DEFFENITLY I WILL GIVE HIM A GOOD APPRAISAL,THAT WILL HELP THE PERFORMENTS OF THE BUISINESS . IF THATS FOR HIS PERSONAL GOALS I JUST ASK HIM TO THINK ABOUT TO STAY IN COMPANY OR NOT