Blocking AIM with IPChains:

ipchains -A input -b --sport 5190 -j DENY
ipchains -A input -b -s login.oscar.aol.com -j DENY

*** NOTE: This blocks ICQ as well! ***

Blocking AIM with IPTables:

iptables -A FORWARD --dport 5190 -j REJECT
iptables -A FORWARD -d login.oscar.aol.com -j REJECT

Information on blocking via other firewalls:
If you would like to block AOL Instant Messenger with another firewall,
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.