After performing a trick for one of my brothers, he always makes guesses outloud. Sometimes they are REALLY close and other times they are no where close to accurate. This poses a question for me. Should you attempt to prove to a guesser that there guess is inacurate(whether they're right or not). Should you simply ignore them and let them guess as many times as they want?
I let the first guess go by without a comment. The second time it happens I pack it in and say "If your a magician go and finish up." When they start to sputer and stammer I will tell them "I will be glad to teach you how to do this effect, Lessons are $100 an hour with a 5 hour minimum fee paid up front. Cash only."
It will shut them up in a hurry. Also I never let a person get close enough to make a grap at me or touch me so they can not grab any ITR or other gimmick.