Well, if you're like me and already have backup solutions running in your Open Directory, you may not want your users using Time Machine, for whatever reasons you may have. In Work Group Manager (WGM), select your groups (or your parent group) and select the Details tab. Then add the following to the details list:
/System/Library/CoreServices/ManagedClient.app
Once you add that in to the Details part of WGM, you can select the menu items and click on the pencil icon to add a string to disable Time Machine from the menu bar. So, click on the pencil icon and a new window should pop up. It should display Once, Often, Always. Select and expand the arrow key on Always, and then click on the New Key button. Select Edit from the drop-box, and name it TimeMachine.menu, then for Type, select boolean, and for Value, select false. Attached is a screen shot. It should look like this when you're done.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20080801104313882