Ferral, using Gnutella or any kind of file sharing is not illegal at all. It is illegal to pass information about users to the RIAA, MPAA etc. Those are simply commercial organizations with no privileges whatsoever. Violating the privacy fits those organizations pretty well though. As you probably know Sony/BMG as one of the largest member of the RIAA is guilty of computer sabotage in more than a million cases.

You should just talk personally with some of the LimeWire users and explain that and why they are not allowed to use it. Treat your pupils with respect and keep civil, then they'll respect you as well. Technical measures will only encourage some of them to find ways around those.