Managed preferences are set up by the administrator of the Open Directory / Active Directory, so short of unbinding your mac from all of these servers, you will have to live with the changes imposed on that mac or keep changing them when you log in.
Do you really need to stay connected to the servers if they are making changes you can't get them to reverse?
Launchd would be a good tool for you to add a script that makes those changes for you when you log in, but that's just automating a workaround and not resolving the issue. Similarly, launchd can be used to implement Managed Preferences on boot as described in this short video seminar. You might have to check into launchd on the mac to see if someone implemented a similar restriction using Workgroup Manager.