1. answers and not questions.
2. positive evidence, and not hypotheses.
For the first, that means that statements like "Don't you think it's strange...?" statements are not permitted. Generally, conspiracy theorists won't provide answers, they will merely provide doubt to demonstrate a gap in your understanding, and then will happily try to fill the gap with a plausible (but usually wrong) theory.
It will start with "Don't you think it's strange/unlikely...?" regarding some aspect of the official narrative, and then follow with statements like "The only way it could make sense is if ..."
I point out that while he may have a compelling hypothesis, he didn't point to any proof of his hypothesis whatsoever. They are much more likely to point out absence of proof in parts of the official narratives, so use this same strategy with them. Positive proof means something actually demonstrated, not inferred.
It works really well. Most of them will fail right at step 1: "You've asked a lot of reasonable questions, but have not provided any proof for your theory."
Providing proof is hard. Coming up with ideas is easy.
Not sure if it'll work with random people though. It may have worked only because I know these people personally.
Edit: One more important tactic. Most of them rely on the "If you're wrong I must be right!" strategy[0] (or rather, they fall for it). So be very open to accepting that the official narrative is wrong. Say (sincerely) "I can believe the official narrative is problematic or even wrong, but I'm not sure I see a reason your narrative is correct. It's quite possible the true explanation is something else entirely."
[0] See the movie Thank You For Smoking for a good reference to this.