A lot of them been so indoctrinated into mistrusting authorities and instutions, that they basically disbelieve anything they say on principle.

And al the evidence, all the scientists telling them they're wrong just ends up reinforcing their belief in some giant conspiracy.

It's sadly been shown in more than one study that changing the mind of conspiracy theorists with reason, argumens or evidence is basically impossible. It's almost a self preservation instict against cognitive dissonance. They were so sure they were right, and now so one is telling them they're not. That feels shit, and it feels shit to accept you were wrong about something you so fervently insisted was true. So their brains basically go into self defense mode, and just reject and attack anything that threatens the shaky fundamentals of their entire belief system. The best thing you can attmept to do is to distract them. Get them to talk and think about other things. When they mention the conspiracy, don't engage, don't argue how they're wrong, they'll just dig their heels in deeper, just change the topic to something else. Force them to spend less time in their delusions. Eventually, if you're lucky, they might gain enough distance to the topic, and stop caring about it enough that they're ready to start accepting how batshit insane those conspiracies are.