@Jaunt said:
"You cannot play PCs" is pretty straight-forward. It might be a bit of a blunt solution, but it definitely works.
It is a blunt solution, and, as demonstrated, it has the unintended consequence of making it extremely difficult to find willing, knowledgeable staff for a game.
If you hire well, however, you won't need such guidelines because you won't need to worry about staff making bad calls.