How can I stop Instant Messaging?

I have a Microsoft NT 4.0 server running Proxy 2.0.

I need to be able to prevent the use of Instant Messaging whether it be AOL, MSN or any other.

Everyone has to pass through this Proxy server in order to get to the WEB.

Is there any way Ican stop the use of instant messangers via use of the Proxy server?