I think you might find some solutions in a great negotiation book called "Never Split the Difference". Written by a former hostage negotiator. If you can negotiate with someone who thinks they are the Messiah and has a gun you might stand a chance online.

The problem is negotiation requires care and a lot of work. It's hard. And in public forums others will probably mess up your progress.

My best guess in a public forum would be to play the "good cop" and quote and defend the person I wanted to persuade. And while looking like I'm defending them, and being their friend, I would put forth an idea that is more moderate and thus move them towards being persuaded. This would be hard to do in the chaos of an online forum like HN or Reddit though.