The best answer more than likely is the one of the ones that tried a pre-emptive strrike against the question being asked to beign with. That would involve audience control, which can be broken down into two parts. A) The attitude of the performer and B) The presentation of the trick itself.
Let's start with A). How is it that you are performing the trick? Are you confident that it is going to work? This may seem obvious but the mnner in which the trick is presented can really affect the way you are going to be looked at and perceived, which includes how you will be questioned. For example, if you aren't presenting the feat as someone who looks as though he truly believes that he performs miracles all the time, but are presenting it as someone who just learned the way to do something neat, you are going to be questioned as such. If there is more emphasis on the "Just watch the magic happen. Don't blink or you'll miss it, and I don't think I can do it again." rather than the "I just mastered this and could do it anytime I wanted." Are you taking the audience down a path? Are you building enough suspense and timing during every performance? For example, when you see the demos for one trick, you'll notice that they are with crowd for a significant amount of time .... for one trick. Is it only taking you 20-seconds to zip through each trick? The reason they are taking their time is because they are making sure that timing is right and that suspense is built correctly so the audience is more emotionally involved creating a heightened satisfaction at the end of the trick. If you present yourself as a professional, you will be treated as one.
Now for the trick itself.I find that if I am performing a multi-phasic routine, there is less likelihood of the question being posed. The beauty of such routines is that as far as the spectator is concerned, they ARE seeing the same trick twice ..... sort of. This why I love tricks with lines in the middle like "I'm going to show you that again, but a little slower." or "Now I'm going to show how that's done" while showing them a different ending altogether. Seemingly, you ARE showing them twice, but now you have produced a different answer as well. One of three things happen. Either they are completely satisfied as they "saw" the same trick twice and got two surprises as well OR they are still trying to juggle between the two surprises in the head and by asking you to do it again, they run the risk of a third option to sort through OR they realize at this point that asking you to do it again simply means that you are going to do a different effect for them so asking is a moot point. Any of these options is very beneficial.
As well, with the "I will do it a little slower" helps prevent being asked to do it again because there is the subtle implication that by doing so, there are admitting they NEED it to be done slower. Now, that idea is often argued about whether it is correct to do or not, but I do agree with the posture it presents the performer and that the implications of insult are so subtle that people really don't pick up on them to the point where they feel that I am outwardly and overtly insulting them. They instead create the idea in their own head that by asking, they would be admitting that they need it slowed down further. But I do respect the opposite opinions as well, so this maybe something to judge with your own beliefs.
As well, I try to couple effects in my head that share similar beginnings or ideas. This way I can incorporate the above idea into a routine. For example, one trick that often gets asked for a repeat is two card monte. And for good reason. It is an awesome, simple hard hitting trick. But I'll say that I'm going to do it again, but I'll do the Daley's Ace Trick (I don't the exact name of it, but most of you will know what I am talking about). So I always have these couplings going on. Everytime I see a new trick, I couple it with something. Now if after just doing the TCM, I get asked to do it again, I simply do the Daley Routine afterwards. The person isn't exactly upset, as I did another trick with a similar ending. The only reason they would be upset is that they don't know the trick to either of them (which is kind of the idea), but they are starting to get that I am probably not going to show them the same trick any time soon. And just to hammer that in, I would follow-up with Garcia's .44, which also is the same idea of trick. So I have let the spectator know non-verbally and without confrontation that I am running the show but also treating the spec with respect as well. Now of course he may see me do it again, but he doesn't know where or when, so I still have the element of surprise.
And if, after all this you find yourself still being asked, the best escape is one with a laugh. The classic mentioned in earlier replies is good. I tend to be slightly more risque, since my target audience is. "Are you kidding? Whatdo you think I am, Superman? I need to recharge and have a cigarette or something. I tell you I think I am almost too good at this for myown well being." I, of course, say this with the correct overtones so that people get the joke with the understanding that I am not doing the trick.
Overall, be sure you have this type of confidence and you'll be fine. And overall, don't worry if you have broken this rule. The best lesson is experience, and we have all been there. We have all tested that rule just a little too much and found our personal boundaries with it. But good luck and I hope this was helpful.