Did you just ask how to... block MSN Messenger? Surely, you must be out of your mind. But alright, we'll respect your decision as long as you remember to reverse your changes afterwards.

MSN Messenger communicates with the .NET Messenger service using either port 1863 for direct TCP connections or using HTTP using port 80. Network administrators can disable access to the .NET Messenger Services through their firewall:

1) Block outbound access to TCP port 1863.

2) Block HTTP access to messenger.hotmail.com.

3) Block HTTP access to webmessenger.msn.com if you also want to block access to MSN Web Messenger.

*Pssst... I didn't tell you this*