From my experience, conspiracy theorists are like everyone else epistemologically except they have different motivations: most people want to believe the world is just, while conspiracy theorists want to believe the world is sinister. Like with most people, you have to watch out for motivated skepticism or what skeptics call anomaly hunting.

To guard against that, you'll have to find a way to make their evidence clearly defined and inescapable. I recommend having them write down all the evidence for their belief that 9/11 was blah blah and then weight it. Here's an example:

• 7WTC blah blah is 60% of the support for my belief that 9/11 blah blah
• Nanothermite blah blah 15%
• Blah blah holograms 5%

Ask them what percentage of that evidence is sufficient for their belief, and then just take the paper and research it. Come back another day and discuss your findings dialectically.

If you're looking for information resources, here's some mixed quality ones from my bookmarks:

• http://www.skeptic.com/searchresults.html?cx=018011741860882051434%3Aalihtmnfi_m&cof=FORID%3A10&ie=UTF-8&q=9%2F11&sa=search+skeptic.com&siteurl=www.skeptic.com%2Feskeptic%2F08-11-12%2F
• http://theness.com/neurologicablog/?s=9%2F11
• http://www.reddit.com/r/IAmA/comments/akrna/i_am_a_debunker_of_911_conspiracy_theories_ama/
• http://screwloosechange.blogspot.com/
• http://forums.randi.org/local_links.php?catid=18
• https://sites.google.com/site/wtc7lies/
• http://www.dalimunthe.com/2010/03/top-ten-photos-911-conspiracy-nuts-hate.html
• http://www.youtube.com/watch?v=t1wQ2BJsgx0