Stopping the communication between nodes is normally seen as a bad thing, because when communication is reestablished you will have to reconcile the modifications on the separate partitions (in your case it would require a 3-way merge). That's why it's not supported directly in Infinispan.
If you just want to run tests to see what would happen, I suggest binding Infinispan to a dedicated network interface and disabling/enabling that network interface externally.
As a possible alternative it looks like JChannel supports restarts, so you might be able to do something like this:
RpcManagerImpl rpcManager = (RpcManagerImpl) advancedCache.getRpcManager();
JGroupsTransport transport = (JGroupsTransport) rpcManager.getTransport();
JChannel channel = transport.getChannel();
Note that this is not a supported scenario and I haven't actually run the code, so it is very likely that it doesn't work and it blows up instead.
I just learned there is a better way to test a network failure: inserting a DISCARD protocol in the JGroups protocol stack.