... someone says they know the secret of the trick, they're wrong, they tell what they think and everyone believes them, but they're wrong.

Tell them to do it... They normally fu*k it up... unless it's gimmicked...

Then what you want to do is just ignore him or perform a funny trick on him so he's humiliated.

dont humiliate or challenge them. that response is many peoples natural reaction to magic. typically its nothing against you. be polite and respectful. it may be difficult but the best strategy is to just say something like "really? you think thats how thats done? ok" and just leave it that way. when you respond like that, it doesnt make you look defensive or angry, and it leaves them with doubts. because of course your natural reaction is to try to prove them wrong. if you just relax and let them keep their theories. you come out looking much better and typically the situation is kept from becoming a problem. Whatever you do, dont attempt to verbally attack a spectator (to humiliate them or whatever).

Just do another trick that looks similar in moves but obviouly disproves their theory. But do not call any attention that you are proving them wrong. Never confirm if they are right or wrong. Many times I'll just say, "wow, that's a neat concept...I think I'll try it that way next time...but I'll have to practice it at home first". You can really say just about anything or nothing. But again, never confirm or deny if they are correct.

If the accusation is fairly far-fetched, give them the cards and tell them to do it. It almost instantly disproves their theory.

Exactly what I do.

Tell them, "Well, thanks for your opinion." Then go on with your routine and ignore any further comments they make. Take the high road and most clowns won't follow. Or, you could say, if you are in a mean mood, "So, when you go out to eat do you tell the chef in the kitchen you know the recipes too?"

just go: ya, sure thats how a did it and roll your eyes, laugh and go on to another trick