You devil, you! You could get a Doublemint gum package that had a spring inside (sort of like a mousetrap) that would snap on someone's fingers when they pulled out the fake stick of gum. We were pretty wild in our youth!
Not only stupid; impossible to enforce. What are they going to do when someone takes a picture, throw them to the floor and take away their phone? You know, from the person who is a paying customer.
But that only looks correct from one location. What we need is a ceiling projector that corrects the image based on the viewer's position. Similar to what they did (spoiler >>) in the recent Mission Impossible movie. Like in the movie, though, multiple people would screw it up.
http://theweek.com/article/index/225856/is-the-world-running-out-of-helium