I have a new approach I like to use whenever I come across a conspiracy theorist. I ask them if there are conspiracy theories they have looked into but do not accept as real. My goal is for them to (hopefully) name one such conspiracy, and then compare the rhetoric used with that conspiracy to the one they do accept as real. Since all conspiracy theories are pretty much the same, I hope to show them that the ridiculousness of the theory they don't believe, also applies to the one they do believe. Sort of get around their guarded pet theory and show the error in the general thinking of conspiracy theorists.
I've only had the chance to do this twice. The first time, I got no answer (this was online). The second time, I came across a conspiracy theorist who actually believed every single theory I could list, that he had heard of. The ones he hadn't heard of seemed more than plausible, according to him. Every. Single. One.
More research is needed in order to determine if this is an effective approach.