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:
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:
I hope I'm not breaking any taboos here. It's been a while since I've come onto the discussion section and I admit I'm not too up to date on the topics.
I'm having difficulty responding to someone who is convinced that 7WTC was brought down by controlled demolition on September 11th, 2001. They're referencing the controlled-looking destruction of 7WTC and various other incriminating looking things. Thermite and thermite waste products seem to come up a lot.
Now, I have definitely noticed I'm confused here. While I hold the opinion that the towers went down because of the planes/fires (i.e. the standard explanation) I have difficulty seeing how the falsity of controlled demolition is the slam-dunk folks seem to think it is. Could somebody walk me through this?
[EDIT: About a million edits later, I have finally worked through the problem with my link: I needed it to be in HTML and not in the comment format.]