In addition to the gish gallop mentioned above, this is also a situation where you could invoke Sagan’s Law, that extraordinary claims require extraordinary evidence. If the person is making crazy claims, for example that there is a group of evil elites that are kidnapping children and stealing their adenochrome, the burden of proof is on that person to provide evidence for their claims - not on you to provide evidence that they’re not true. Eg.

Person A: Blah blah Pizzagate Blah blah, what do you say to that?! Person B: Sagan’s law - the burden of proof is on you to prove that what you’re saying is true. I’ll respond only if you provide me with evidence of what you’re claiming.

Of course this works better in a formal debating situation than in an argument with a crazy Qulter, but.