Have you had the experience of putting privacy sign on your doorknob at a hotel, and then they ignore it, come in and make up your room? My last hotel used a technique that seems a bit hokey to me, but it really ought to work: you stick the privacy sign right into the slot for the magnetic card to open your door. The privacy notice seems precariously held in place, but you know that a cleaning person is certain to notice it! They have to take it out to put their own card in and open the room.
(They made up my room anyway. Argh!)