See signature.
Well, seriously, I just try to ignore it, or give up an amused sort of laugh. That seems to make them indignant, at which point you either defend yourself and proceed with the heavy machine gun, or walk away. The latter seems to happen roughly three thousand two hundred and seventy six times out of three thousand two hundred and seventy seven anyway, so it's pretty easy to ignore in the social situation. I still walk off fuming, and concocting malicious schemes about beating them repeatedly about the head with a stone hammer labled "IGNORANCE" in large black runic text, or some such nonsense, but i find it usually goes away pretty quickly. Emotional arguments, are, in my experience, not worth having, and I'd say that slandering someone by saying they don't exist is pretty high on the "things-not-to-say-to-that-nerd-in-the-corner-who-looks-like-he's-sharpening-his-teeth-into-the-integral-shapes-of-three-dimensional-calculus-equations-(calculic?)-in-that-book-there-yeah" list.
Still, we can be just as intolerant, I find. Merely human failing, albeit an unpleasant example in this scenario.
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CARPE PECTORIS!!
There is nothing about these perplexing and morally insensitive humans that cannot be solved with the aid of a heavy machine gun.