Don't give them power over you. Walk away or remember that they aren't in your skin. You know better.

Last resort...get a really good, really simply term book for them to read. If they read it; answer questions graciously. If they refuse to read it; remind them that not every person does what they "look" like they can do. Jeffry Daimler, I'm told, looked perfectly "normal."

It's sad, really.