First of all, I wouldn't call such people hecklers. The audience is supposed to be faced with the puzzle, "How does he do it?" It's only natural for them to guess. When David Copperfield does "Flying" do you think that there is a single person in the audience who isn't looking for wires?
You have to wing it based on what the "wrong stuff" is. If they say it's up your sleeve, you can roll up your sleeve. If they say you have trick cards, you can show them the cards. I remember once doing a super heavily gaffed monte routine, and just when I did the packet switch for the ungimmicked cards, somebody said that I had trick cards. It was a pleasure to show them each card very carefully, front and back, to dispel their notion. The humor is, of course, their notion is 100% correct, that's why it's great to stay one step ahead of the audience at all times.
Another example is the pen-through dollar trick. I used to believe that signing the bill was superfluous, but then I found way to many spectators saying that I somehow switched bills. These days I make them sign. It seems strange to magicians--we know the circumstances that a bill can be switched, and the pen-through-dollar presentation simply doesn't fit the model. But to the audience, you are capable of superman-like sleight of hand, and any kind of sneaky switch is possible in their mind.
Sometime they'll say something which is so incredible, but you can't explicitly prove that they are wrong without repeating the trick. A good example is the invisible deck. "You slid my card out from underneath while you were spreading through the deck and flipped it over." At that point you have no choice but to just make a joke of it. "I did that! Really? darn, I'm better than I thought!"