you can block those through the server itself, cant' be blocked by mails ... otherwise ... I can create a new account and use it. You can block it either using a proxy [such as ISA] or a software that will use ISA as a plugin [Smart Filter] ... some are still happy with the old school stuff and are using Surf Control ...

If you have a Juniper Router, you can create groups, add users to this group and then block those chat programs ... a CISCO firewall can do the job also. 

 

------------------------------------ Do not dwell in the past, do not dream of the future, concentrate the mind on the present moment. (Gautama Buddha)