"Dude, I just realized..." [insert conspiracy theory here]

If indeed you are sane, you need to reply immediately that, "I don't give a shit about your stupid fucking opinions, or your goddamned speculations based on a staggering ignorance of our history and human nature, fuck you very much." This won't stop them. 

Stupidity needs an airing and wants to parade itself: "No, really," says the asshole that flunked out of history and barely passed civics. "This is REALLY important, it's the truth!"

"No, really, I'm not fucking around--I don't value your opinion about anything."

Feed a cold, starve a fever dream. When you respond to a conspiracy nut, you're giving them oxygen. The same goes for political opponents. Let them suffocate alone, in silence.