Hi -
If you would like to block AOL Instant Messenger, then you have to block 2 things: the port on which AIM operates, port 5190, and/or the server to which the majority of all AIM clients connect: login.oscar.aol.com. Use access-list either on firewall (PIX) or router.
Also for more info on stoping other IM services then check the following URL:
http://testweb.oofle.com/messaging/index.htm
Hope this helps - Jay.