Well true to a point. If they have it posted at the service desk, and they catch you at the service desk, you're boned. It's posted on the employee corkboard near the bathroom and they get you coming out of the bathroom, you are boned.
The other is if they have asked you to leave before and catch you again, you're boned, because while it's only a warning when you haven't seen a sign, being verbally told by management is the same thing.