How to simulate socket connection loss?

In one of our client-server applications, we need to check the behavior of the application when socket connections are lost for various reasons. For this test, we need to simulate a program that can interrupt connections on a specific port on the server several times. Thus, we can check how connected clients react to such disconnections.

Please suggest me if I can do this in my code, or I can use it with any free software.

EDIT: We know the port on which we need to block connections for a few milliseconds or seconds.