I've seen it here and there and have extensions for that. It's no Reddit, but far from perfect. Especially once you start poking at a few specific topics (some expected and practically universally controversial, others more "tailored" towards a technical community).
You can collapse threads, and the setting will hold until the post is about 2 weeks old (at which point it's locked from further comments).
I am doing this with ... somewhat increasing frequency.
It's probably possible to use Greasemonkey or similar to provide client-side JS for blocking users, though that would be cumbersome. Some of the alternate HN front-ends may also offer this.
I consider that a pretty important feature for mental health reasons.