My usual process is to try to get them to explain why their position is right, find out where in their argument isn't sound, and point that out as a counterpoint. People are much more open to considering other positions if they realize there is an issue with theirs, and usually when they're not listening to reason or going with a conspiracy theory of some sort, then they're likely pretty closed off to opposing viewpoints. Usually the people who believe this stuff still do believe in some scientific principals and evidence-based arguments, but they've convinced themselves that their evidence or principals are somehow better than those of the opposing viewpoint.
That said, it's a resource intensive method as you actually have to pay attention to their arguments and research them in some cases.... so for people who I don't really feel connected to, I generally don't bother. That's when the smile and nod or leaving it with an awkward "I disagree" will do the trick.
For especially elaborate conspiracies or especially simple explanations, Occam's razor could be a good enough argument to be right, but again it's unlikely many who are heavily invested into their version of reality will just accept that their position is flawed.