You can't reason someone out of something that he didn't get into by using reason in the first place. That's why it's so difficult to refute conspiracy theories with logic. This is especially relevant given the way that conspiracies spread in the Internet age.
It starts out with a cuckoo theory promoted by crazy people and integrity-free professional con men who pump every conspiracy that comes down the line. The crazy people push the conspiracy theory because they're paranoid and don't understand how the world works while the sleazy hucksters promote it because conspiracies sell on the web. Eventually, as the conspiracy starts to draw a crowd, more imitators pop up and the crazies and flimflam men create a vast web of references to each other.
Meanwhile, the people who know better generally ignore the topic altogether because they don't want to draw attention to it. Unfortunately, that tactic doesn’t work anymore. In the bad old days when there were only a few mainstream "gatekeepers" who controlled the flow of information to the public, it could be successful, but it's a non-functional strategy in the age of New Media.
So, when the average person starts to research a conspiracy theory on the Internet, he finds an endless array of conspiracy websites, all linking back and forth, while the doubters largely remain on the sidelines. After seeing only one side of the story, many people who don’t know any better conclude, "Well, I GUESS this could be real." Next thing you know, 35% of Democrats think George W. Bush was involved in the 9/11 attacks and another 26% aren't sure.
One thing that most people seem to miss is that these conspiracy theorists almost NEVER present any sort of coherent theory. They just ask questions, point out discrepancies, and demand that every single bit of uncertainty be answered to their satisfaction or else, the conspiracy MUST BE true. This is, of course, perfectly ridiculous. No matter what the subject is, there are always disagreements, loose ends, and questions that remain unanswered.
Just to prove that point while I give the Truthers a taste of their own medicine, here are ten questions about the 9/11 conspiracy theories. The next time someone asks you wacky question about the “government conspiracy” behind 9/11, hit him back with a question. Granted, it won't change his mind if he’s a kook, but you just might reach a few of the casual conspiracists who haven't fully embraced the crazy yet.