In an interview you have been asked: As a manager how do you handle interference in your department?

Limit the visitors on the basis of priority in case of interference from External sources. Internal interference, restrict at the first instance itself if, it from subordinates, if it is from peers or superiors hear it with one ear and leave  it over the other.  unavoidable circumstances report exactly in the right place using diligence and choice that best suit the circumstances. In certain occasions non-reactions itself will reduce or solve the issues.

Taking a firm stance to put red line to him to not dare again to interfere in my work.

I agreed with Mr. Vrindavan answer.

1. Identify the problems.

2. Meeting with team members.

3. Develop policy on this issue.

I agree with Mr. Ayoub and Mr. Moorthys' replies for restricting interference.

I will say that, I will refer to the company policies, standards and procedures for handling this kind of interference, then I will discuss this issue with my supervisor for any mitigation tools to be deployed.

That means most probably there's a gap within the procedures and responsabilities 

or they're not defined formally , or a miscomprehension of the responsabilities.

Meet with the concerned people , clarify and write the duties.

There are two ways to the issue. Reading through each person/department's roles and responsibilities may help define the borders. This is helpful were the interference was a result of ignorance from either side.

However, reality is a lot more complex. In difficult situations, there is a need to demonstrate emotional intelligence. Understanding how the organisation works will also help understand the significance of and the impact of any interference. 

The end result is to encourage involved parties to reach a compromise such that they still have the needed motivation to carryout their duties. 

As a team we have to work together to spear head progress and not to criticize others, at some point interference should be welcomed as a improving tool but if this happens on daily basis something is wrong with the criticizing person one must investigate rather than rocking the boat.

Very good question.

Being manager, I will make sure following,

- Proper business process is in place and followed accordingly

- Effective use of communication tools i.e. online workflow management system, e-Mails, Tel etc.

- SPoC [Single Point of Contact] for all the operational level communication is in place

- Department level understanding and co-operation in streamlining business process 

Firstly, is the person authorised to interfere with the department activities, if yes, I would request his specific purpose/motive via official channels and Give the appropriate response for averting further interference with a Due note to authority which delegated hims such a duty. If no, then I would first assess motive requesting him to use official channels, and if the motive is undesirable then judging by the stature I would find a soft/ hard response to keep him at bay and report the same to the concerned authority.