Windows Mobile 5:
The easiest way to un-register a Windows Mobile device is to go into the GO!NotifyLink Prefs on the device itself. Then go to Account Settings, click on the Menu button in the lower right-hand corner, and select Delete Account. The device is now un-registered.
Another way that you can remove the registration is inside the Admin Console. Select User Administration --> Select the user --> User IT Policies --> Security --> Click on "Clear Device". This will send a kill message to the device to completely remove the registration and all associated data.
**Please note that if you choose the method through the admin console you will then have to clear all pending messages for the user's account if you try to set up a device again because this leaves a wipe message pending for the account. To clear this message you would go into User Administration --> Select the user --> Edit User Device --> Clear All New Messages