Tom~
You are correct in your assessment that we can't stop people from buying our work and making a pattern from it, although I can't imagine why anyone would do that as the pattern itself is almost certainly less expensive then the actual cutting. If not you really would need to take a good look at your prices.
However, as a "professional" exhibitor at shows I can tell you you ABSOLUTELY CAN prohibit anyone and everyone from taking pictures in your booth or of your work. At the risk of delving into the whole copyright controversy.... All work created by an artist is copyrighted as soon as it is completed. If your work can be copied by taking a photo of it you can refuse to allow it to be photographed. I have seen people who post signs in their booth that all of their designs are copyrighted and that photography is prohibited. How you would actually enforce that however is another matter.
It is an awful shame that folks resort to this kind of tactic, but as stated earlier even big business tries to get away with it.
Hope that helps
Jeff Zaffino (shadez6)