How to block AOL port 5190 in 2k server

I have windows 2000 which has two nics. One external and one internal. There is no software or hardware firewall but I am using NAT for the internal nic. How then can I block users from the inside so they cannnot access AOl? I saw the ip filtering option in windows but this is only to allow, so I would have to add all the ports and i’m not sure how many I use. Is there an effective way to block this port with standard windows 2000 server software?