You'll never "reach" the hardcore conspiracy theorist because they aren't really interested in the overt subject. They're actually trying to achieve something else and this is the chosen method. For example, the Pearl Harbor disaster is said to have been a case of "just let it happen" or such. "FDR knew about it and let it happen to get the US into the war." The deaths they beat their breasts over, and the material damage they find horrifying, are just a way to slam FDR, another way of proving he was an evil monster. So it doesn't matter how many fatal flaws to their theory are presented, they just don't care. If they can get one more person to believe, they're happy.
The way to counteract this is with the facts, of course. The Myths of Pearl Harbor is a page I put up while at Purdue. It's used by historians at the Army and Navy historical centers, they refer questions about any conspiracy to that page as a starting point for further research. Saves them time and they don't have to take an official "stance" on the matter that the CTers can use to further beat their drum.
The way to counteract this is with the facts, of course. The Myths of Pearl Harbor is a page I put up while at Purdue. It's used by historians at the Army and Navy historical centers, they refer questions about any conspiracy to that page as a starting point for further research. Saves them time and they don't have to take an official "stance" on the matter that the CTers can use to further beat their drum.