The only accountability for a niche forum somebody else runs elsewhere is that you tell people how they suck and to not go there. You can't send the cops on them like how you could call them on a coffee shop where they're doing illegal discrimination.
If other people agree they suck they'll block that community and leave for another that's better run.
As a moderator of computer security forums (cryptography specifically) who deal with both cranks and aggressive spammers - you just can't do without bans.
The only long term solution is to ensure there's always more than one community with separate leadership, so there's accountability for mods through pressure from the users. The threat to leave must feel credible.