MSN Messenger Live is particularly difficult to block. Not only does the application use dynamic ports making it difficult to block, savvy users can always use the web based version bypassing any blocking you’re doing. I use hardware blocking to try to prevent this at home. A product called iBoss effectively prevents all IM usage on a computer by computer basis, but it requires a monthly fee and can slow down network access. Sean’s note: If you want to eliminate a program from a computer, you will have to remove it from the machine if it’s already there, using the Add/Remove Programs dialogue box — but since that won’t prevent someone from simply reinstalling it, the second step is to only allow other users to access that computer in limited user mode. That way they’ll need to get the administrator password in order to add or remove any software…and if you don’t want them loading chat clients back onto the machine, you just don’t give them the password. But be warned: that won’t stop them from using the web version of the Messenger client (http://webmessenger.msn.com/). You can try blocking that site on your router, but there’s almost always a way around such blocks…it could become like a game of whack-a-mole.