Calling a conspiracy theorist a poopyhead won’t help either (they thrive on perceived persecution). The only thing I think Might help (and it’s a longshot) is along the lines of Socratic Method.

Or even just avoidance of trigger topics and only talk to them about normal everyday stuff. That’s what I try to do with my mom, even if I’m not changing her mind on anything, at least I’m not actively pushing her farther down the pit and away from me/society.