I would respond by assuring the person that his understanding
of the scientific thing called a "theory" is all wrong, and that he
needs to do some research into the Scientific Method before he
maligns, denigrates, and insults scientists again. I would put some
real effort into convincing him that he is just shooting his mouth off,
and seriously doesn't know what he is talking about.
I know that this is the worst possible way to respond.
But these goons irritate me, and the easiest way to push my
anger button is to say something that has "only a theory" in it.
Also, I've had a bad day.