There is a large red stain on the entry carpet at the front door. I don't know where it came from, it just was kind of there one day. Creepy right? Whenever we get salesmen or proselytizers I stand with the door open and shift my eyes between them and the stain as they are giving their pitch. When they are done I say: "Come on in, stand right here," - gesturing at the stain - "and don't move." 

Usually they decline.

This works even better if I happen to be holding skin moisturizer or duct tape at the time...