Other option, Louis Theroux that shit

Just don't react, but keep asking "why" type questions, again, just acting interested, like you think they could convince you if they're just explain it better.

Make them try to argue their own way into a hole until they're either so pissed off they drop it, or they start to disbelief their own thoughts.

It works cause you're not arguing against stupid that way, you're making stupid argue against itself, and nothing beats that