Blocking yahoo messenger and msn chat

[sim]

I have a users inside my nework who used yahoo messenger
and msn to chat in the internet. I have a firewall inside
my network but my problem is I can't find the IP address
of the server that yahoo used for thier chat software. Can
anybody tell me on how to block these users from chatting
over the internet within office hours? Thanks a lot!

 

[Me2aswell]

one method is to type in the run box

gpedit.msc

look for software settings and diable them. That woked for msn as for yahoo
I set group polices to prevent loading software.