If you give us more details about your network setup, we might be able to help a little better. One thing is for sure though, AIM can adapt itself to pretty much
any network configuration in order to connect. The AIM login servers listen on pretty much every port [I've seen people connect using ports 13, 80, 445, 8080, etc.].
Your ideal best bet would be to set up some kind of packet analyzing machine in order to check the contents of each inbound/outbound packet (thus severely slowing network access, unfortunately) and if it saw AIM packets, to drop them.
Of course, even then, someone could SSH tunnel to an outside computer and run AIM through the tunnel.
