This is the internet. People are going to say stupid and cruel things because they are nasty ignorant fools or because they take sadistic pleasure from your reaction, i.e. they're trolls. So like others have said, your main course of action is to ignore them. Alternatively, you can learn a thing or two about trolling yourself, understand the mindset, understand how to elicit a certain kind of reaction and who is most likely to produce it, and maybe even turn the tables on these people a little bit (or have such a profoundly superior understanding of the art form that their primitive pokings and proddings register in your mind as a kind of child's play that is not even worth your attention.)
This is coming from someone who has trolled and learned of trolling in (I think) more artful, interesting, fun, non-stupid ways. No judgement please; you have no idea what I did or why. My point is only to say that, well, there's more than one way to solve a problem. :O Also this is a bit of a roundabout way of saying that this person is not worth your time, even from a pro-trolling(ish) perspective.
EDIT: And yeah, widespread misconceptions about autism play into it as well (as far as this particular incident is concerned), but others have covered that already. XD