Disable Windows Messenger in Windows XP

Does anyone know if there is a way to completely disable or get rid of Windows Messenger in Windows XP? I work in a school division and we have a XP lab in each elementary school. We disabled it in the profile most students log in with, but if someone else logs in it trys to start. We have configured our firewall to block instant messaging programs to keep students from chatting, so when Windows Messenger trys to open it bogs down the firewall. Yesterday it tried to start and it created 200open sessions to the firewall while trying to start! any ideas?