Good point ShaniFaye. Maybe an ACL at the firewall to disallow all traffic to and from the AIM port ranges. Unless AIM express runs over port 80. In which case, restrict the site. I'm sure there will always be a way to get around it but hopefully the kids would give up after being met with such resistance.
On another note, take a look at <a href="http://www.faronics.com/">Deep Freeze</a>. If they install AIM, walk up behind them and restart the computer. Bye bye AIM.