Homesync screwed up!

If it's re-appearing every login, you will have to fix it in Workgroup Manager. Make sure you've set the Mobility settings to never... EACH TAB, especially the tab that says Show in Menu Bar. Then log back out and in on the relevant workstations. I'm assuming they are bound to the directory and you used Workgroup Manager to set the setting in the first place?

A good trick for seeing the defaults value for anything that Workgroup Manager can set via its GUI, is to make the setting, click done, and on the main Preference view (where it shows the Mobility and other icons), click the Details tab and you can view the entire manifest you have configured. For what you're looking for, the domain is com.apple.mcxMenuExtra, the key is HomeSync.menu and the value is a boolean, so in theory, the appropriate defaults usage would be:

defaults wrote com.apple.mcxMenuExtra HomeSync.menu -bool NO

But again, if it's re-appearing every login, that's only gonna satisfy you until the next login.

Then you've got a binding or caching issue. Perhaps remove and re-add the directory to the client, and/or flush the cache: http://support.apple.com/kb/HT3540?viewlocale=en_US&locale=en_US (Use the 10.6 version)

Sometimes I even find deleting /Managed Preferences/ and /Library/Preferences/DirectoryServices to be helpful as well.

Good luck!

One way to check for sure to see if a workstation is seeing it as a managed setting it to open System Profiler, scroll to Managed Client in the left sidebar and see if there are any settings there being pushed. If so, that has to be resolved. If not, command-dragging it off the toolbar should get rid of it. Maybe make sure it isn't still a Login Item for those users too.

Glad to hear it's sorted. If your workstations are Lion are greater, you should definitely consider switching to Profile Manager. I get the feeling Workgroup Manager and the old style only exist to support those older clients.