Some ideas:
—reputation system? As in: users could filter users whose rep (as tallied by mastodon.social) is under a specific amount?
—invite-only signup required to contact people on mastodon.social. Possible to get around, of course, but it'll be harder to spin up an account in minutes if you have to get an invite from someone with a good reputation first.
—ghost bans: Mastodon admins can silence users without users knowing. Keeps trolls busy, and they won't think to make a new account.
@geekylou Somebody else raised the same concern. I'd say you have to have human reviewers at some point—barring some super-smart Google AI, this is not a mechanism that could be trusted to be fully automated.
Ars Technica had an interesting article about how Microsoft handles reps on XBox: https://arstechnica.com/gaming/2013/07/microsoft-gives-a-damn-bout-your-bad-reputation-on-xbox-one/
Maybe users of a certain reputation—like on Wikipedia—could be trusted as reviewers as well.