As Ali said, a good one is "Do what?" When they tell you what you just did, laugh and say "What? That's silly. No one can do that." That gives you a perfectly valid reason not to do the trick again. After saying that, walk off so they can't badger you with more questions. If they follow you, I suggest you ignore them unless you can come up with any other smooth lines. If they prevent you from leaving or keep asking questions. Just continue your story that you have no idea how the trick is done so you can't answer any more questions.
That's one way out of it!
I do that! I did that with my b2-bomber. After doing the first bit, the guy told me to do it again, so I did the second bit and he asked me to do it again. So I said "I know" and handed him the examinable bit "what did you want me to do?" "Stab the pen through the paper" "well you can do that, stab it through, then what" "now take it out without leaving a hole". Then I rip it out (because they've already stabbed it through normally" leaving the biggest hole possible and say "You're crazy, noone can do something like that". Has the bonus feature that the audience member's just stabbed a pen through his/her own piece of paper and are now feeling like a right muffin.