[calani]

The MAC is reported directly from your NIC.. your network card, so no, unless you can somehow mask a responce from the NIC, you can't alter it.
Also, it would be -easy- to ban an MAC address. You have the client check the MAC of the host and send that to the server, which would then check it against a list of banned MAC's, as well as the IP it came from. If either are banned, then bang. connection closed.

simple enough?