Rarely are there ever people who stare at my hands or try to get me off my game. As i read your question again, what I'm obtaining is that you want some "trick" that's going to "fix" a spectator. No one will ever be able to straighten out a spectator out by doing just a "trick". Would you like to take a gander as to why? Because, magicians don't do tricks; dogs and hookers do tricks (especially the good hookers, man, do they do tricks!). Magicians perform effects by using proper apparatus AND (and this is the important part) utilizing presentation in order to: 1) magnify impact 2) secure proper understanding of what has occurred 3) a myriad of other important things that i will let you decipher yourself.
As usual, RealityOne is correct. You need to work on presentation in order to become a better magician and include patter to take the spectators mind off the method. However, if you are using patter and this problem still arises, there are one or both of two things occurring: 1) the spectator has had some experience that has molded them to believe that they can treat an entertainer in a negative fashion. Or the more likely of the two: 2) you're not a stable character. Period. You need to be a dominate character in your own goddamn show. You don't need to be offensive to keep them quiet. Sure, you can refuse to perform or simply ignore them. However, both of those "solutions" contain a rather large fallacy.
If you refuse to perform, well, that's just a bad situation waiting to happen. You can't get on stage, perform like a professional and then when you have some bastard in 3-15 yelling at you, just say "f**k you, the show is over." and walk off. No matter what setting you perform in, your spectators are making an investment in you. Simple.
The other popular answer is to simply ignore them. but, does that method work quickly or effectively? No. So why waste your time attempting it? For example, there's this ass***e that's been making some mildly offensive comments. You ignore him and have been for the past few minutes. Problem solved? No. what happens when he realizes that what he is doing isn't working? Chances are he won't stop; he'll move on to more direct applications such as "you're a fag", "let me see that deck" or my favorite, he'll slap the deck out of your hand to gain some sort of attention. Are you going to fold and take that? I sure hope not. Just my two cents, or three...