If the project managers are opening and editing the actual files from project server it is possible to create the scenariao you describe, but it requires programming in Visual Basic to pull it off. It is possible to detect who is attempting to make the change, and then reject the change. This can even be done at an assignment level in a task usage or resource usage view using the Visual Basic that is already built into MS Project.
With respect to Project Web Access, I haven’t had enough exposure to WPA to know what is possible. Do your project managers open the actual schedules to edit, or update using Web Access?
