Run a proxy, only allow for std http (poert 8080 from memory, but probably wrong). Block everything else. Put a download upload limit of 300meg a month on each user account. That should sort it out pretty quick.

Alternatively Put a sniffer on the line, and remove anyone's access whom is obviously using peer to peer.

Tell em to pay for their own bandwidth.

Probably easiest of all is send out a message you are giving the MPAA and ARIA free access to the server records linking IP addresses to user logins.