Mark --
There is no default method within Project Server to prevent project managers
from using local resources in their projects. Therefore, it you wish to
prevent this, it would require you to make this a training and performance
issue. Train them how to use only the resources available in the Enterprise
Resource Pool and then expect them to perform accordingly.
Before making this an issue with your PM's, however, it might also be wise
to find out WHY they are using local resources. For example, they might be
doing so to track the time of outside contractors who are working on their
projects. If this were the case, then creating additional Generic resources
in the Enterprise Resource Pool might address their need and solve the
problem. Just some thoughts. Hope this helps.