The problem is that there are so many other programs that run AIM, like trillian, gaim, etc....
What I did (before we got a layer 7 filter) was block AIM using
Snort. You can set it up to send a RST packet every time it sees an AIM packet, therefore never allowing it to fully connect (PM me if you want to know how to set this up). Unfortunately, this blocks the whole network, so if you want to allow other people, this may not be the way to go (unless you want to set up SSH tunnels for those people).