Well, that's where moderation comes into play.

As much as people hate the idea, you need strict moderation to keep a given platform civil and on topic. To do that you need either robust tools (which lemmy does not have yet), or moderators able and willing to put in the time to keep things on track.

When a forum is run with a low tolerance for incivility, it will eventually become less of a target for jerks.

I catch hell any time I say that because people seem to believe (in spite of a millennium+ of evidence otherwise) that the default state of discourse is friendly and orderly. It simply isn't. People are assholes. When they have the veil of quasi-anonymity, there are large amounts that won't even pretend not to be.

I've seen it happen over and over again since the first html chat I was involved with in the nineties. Even before that, but when you were dealing with pre aol era internet, it was much less of a problem because of the barrier to entry.

You want friendly, chill communities, you have to pick between firm moderation with large and open numbers of users, or light moderation and limited access/numbers. The middle ground is just too open to bad actors.

On reddit, I saw strict moderation change a sub. I saw subs go from constant flame wars and nastiness into a fairly relaxed vibe in a matter of weeks just with active and mild moderation. With stronger moderation and clear community rules, you really can maintain a great community with only bare minimum randos stirring trouble.