Blocking Instant Messenger

You could do both; independs how educated your user are.
If you block port 5190 no one should be able to connect. However in AOL IM you can change the port number. So if your users know about this they can bypass it.
You could however block accessto the AOL IM Servers on any port. That way your users will not be able to bypass anything.

The best way to block AIM from working within your network would be to block:

login.oscar.aol.com

If they are not able to locate the authentication service they will not be able to connect. If you do a search on the web you can find ways to block chat programs like AIM and ICQ. If you need any further assistance please contact me.

You can find a list of port numbers used by instant messaging by going to the following website:

http://www.practicallynetworked.com/sharing/app_port_list.htm

However, I would also block the login.oscar.aol.com as mentioned above

Hello,
I dont know what firewall your running, but you should be able to block access to the executable. ie: for MSNIM, you would block msgsrv32.exe from accessing the internet. If your on a LAN, that would block ALL the other machines at the same time, since the program name is the same on all of them.
You would have to do this for each instant messaging program of course.
HTH
E.D.