Simulating host unreachable - how to achieve/implement it

Here is my scenario:

A is a provisioning server and B is an client. Whenever there is any change in B's setup, it uploads the appropriate config file to A.

I am working as an automation engineer to automate it. One of the scenario says to disconnect A from network or stop the server A. perform some changes to B and make sure that B failed to upload the files to provisioning server A.

To automate it, the simple way to stop the server A and do the appropriate actions.

Since A and B are also used for other purposes by other parties so I can not either disconnect A or B from network OR stop the server at A.

So, I am looking forward for any solution so that I can simulate the host (provisioning server) unreachable scenario. So when B will send an update to A it will fail but in actual A is running as usual.

Please suggest me some way to achieve it.

I am using Perl as a programming language but I am fine if solution is available in other language.