Just let individuals ban whoever they want from THEIR view.
If you want to be super-fancy, you could then see if some account X is banned by many of individual users from appearing on their feed, and give individuals an option to have those automatically banned from their own feeds after some threshold percentage.
So, if X is a jerk/spammer and individual many discussion group users have banned them (from their own view), give users the option to automagically have X banned from their own feed too once they hit say 10% of other members banning them.
This off-loads bannin a little, and as long as individual users have the option to check which those "auto-banned" are and e.g. except them from being auto-banned for them, it still maintains freedom.
In HN with showdead etc, I've never seen any "dead" comments that I couldn't just have as regular comments and just ignore on my own...
https://en.wikipedia.org/wiki/Brandolini%27s_law demands moderation. Community and society demand moderation. Hell, I'd even go as far and say physics demands it. The internet breaks our ideas of social norms on moderation by taking distance and anonymity and shoving them in the same place all at once. And much like if you take groups of conflicting fundamentalist religious groups and put them together, the enviable violence outbreak affects everyone around.
I didn't say to expect they'll behave like grownups in that they wont post anything immature, bad, etc. I said "treat people as grownups", that is, as capable on seeing something they don't like or find offensive or whatever. And if they're not capable, that's on them.
So, if a discussion becomes a flamewar with "thousands of posts", so be it. Members can always ignore it.
So, if the thousands of posts are from the same small number people (over-posting) and others find those annoying, then can chose to invidividually to ban them, or snooze them, or not.
But, if the thousands of posts are by thousants of members (and not bots), then why shouldn't they be left to continue to post and discuss this way, even if its a flame war? They're having fun, and others can ignore or ban them.
Now, if they verbally abuse someone though (e.g. threaten their life, dox them, and such), well, that could be moderated and members who do that could be banned. The rest of opinion, whether deemed controversial, unpopular, misinformation, or bullshit, can stay.
I don't care much about "Brandolini's law". Who is the arbiter of what's bullshit and why are they? The moderator? Well, that's tautological (they're arbiter of non-bullshit merely because they have the power to moderate).