I know I've had my fair share of ALL of those questions at shows.
My favorites are the illiterate herp experts...
"What kind of gecko is this in this deli cup that says Leopard Gecko? It says $50 clear as day, but how much you want for it?" I have actually had people walk over and say, "I see your leopard geckos are $20/ea. How much for two of'em?" My usual answer is $50!
And Rich, Doug is not the only one that's gotten the "Why does this gecko have legs?" question. I got it at the Atlanta show last year and had to get my hearing checked. I mean, I just wanted to be sure that I actually DID hear the stupidest question on earth.
Also, when it comes to children two things work best...sack of oranges...nuff said there
And if they are at my table acting like little morons, I just point to the biggest snake/lizard/tortoise in the near vicinity and say, "if you tap on that again little boy/girl, I'm gonna feed you to THAT!" I think I'll take one of my big boas JUST so I can do that this year again. Surprisingly the parents usually think it's pretty funny(assuming they are there in the first place).
You CAN'T do a show and deal with idiots for two days without having fun with them and keep your sanity.
As far as not selling to certain people, I just do my best to either ignore them or just shuffle them along. There is not much of a scene they can make if you don't pay attention to them. If they ask a question, I answer short and sweet then go to the next person. If they ask, "Is that a male?" I just answer real curt without even looking at them, "yes" then start helping other people in my usual polite way. If they ask another question after that(usually they leave at that point), they either get ignored completely, or get another string of short and cut-off answers. I won't lie to them, but it usually doesn't take too long for them to pick up that I'm not exactly amped to help them out.
The only times I've flat out told anyone they couldn't buy from me were little children that were unsupervised and had no business with an animal bouncing around the table wanting to buy this and this(but of course there is always SOMEONE else that will sell to them). I've even had them tell me, "well the guy over there..." and that's about all they get out of their mouths before they're told "then go over there."
But bottom line, like everyone has said, if you don't want to/feel comfortable selling to a specific person, DON'T!
Cheers! And see hopefully some of you in Atlanta!