I don't have a complete answer, but I think part of the answer is "friction". I think the lesson of the original Usenet, email, Twitter, and Facebook is that if you allow any person in the world to effortlessly communicate with any other person in the world, what you get is a whole lot of noise. Even systems like Reddit have problems with "brigading" where, because somebody with a Reddit account can interact with any subreddit at any time, people in one group can talk amongst themselves and decide they're all going to visit some other group and harass them.
At the same time, it's incredibly valuable to be able to communicate with and learn from other people with different perspectives and experiences, and not to be discarded lightly.
And so, friction. Don't make communication impossible, just put up some hurdles to clear. Maybe that's rate-limiting for sign-ups or posts. Maybe that's an "enforced lurking" period where you have to spend some time reading before you can post. Maybe that's a "sponsor" system where a new member must be invited by an existing member of good standing. Maybe that's a sign at the entrance of the alleyway saying "no tourists". Maybe every tomo community should be able to choose their own.
I don't know what the correct answer is, or if there even is one, but I'm pretty sure friction will be part of it.