In our release process, we have a sanity-check step where one agent (the release manager) will give approval on whether the fix for a given bug is good to go to production. This is currently a virtual user in our implementation of FogBugz 8, and we want a way to lock any cases that come into this queue. Basically prevent anyone from taking cases out of this user's queue: only this user should be able to assign the case. This way, nothing gets out to production without approval.
It's possible to restrict everyone except that user and Site Administrators to read-only access on a project-by-project basis, but there's no other way to keep your normal users from assigning cases out of that Approval User's queue.
However, you could potentially write a BugMonkey script (here are some helpful scripts people have written) which either makes it clear that the case belongs to the protected user or potentially even hides the 'assign' button when looking at such a case.
Another way this could be achieved would be to write a small plugin that inspects case changes and when it finds non-allowed behavior (ie. someone other than the Special User re-assigns one of the Special User's cases), it re-assigns the case to the Special user (basically un-doing whatever wrong action just took place.
It seems a bit hacky, I know, but there are not many other options for enforcing specific behaviors to lock things down.