From the documentation (I don't have a domain setup so I can't be sure if this has changed):
To cancel the service for a domain:
1. Sign in to Admin Center ( http://www.yqcomputer.com/
) with your Windows Live ID (The user
name and password that you use to sign in to any Windows Live, MSN, or Office Live sites and
services. If you have a Passport Network, Hotmail, or Messenger account, you can use it as
your Windows Live ID.) .
2. Under Your domains, click the appropriate domain.
3. In the left pane, click Domain Settings.
4. Click Cancel Service, and then click Yes.
Microsoft MVP - Windows Live Messenger
MSN Messenger/Windows Messenger
MessengerGeek Blog: http://www.yqcomputer.com/
Messenger Resources: http://www.yqcomputer.com/
(c) 2008 Jonathan Kay - If redistributing, you must include this signature or citation