I think web-of-trust is underused for spam and abuse prevention.

e.g. there could be a pretty simple "endorse" button for each account, which is basically saying "I vouch that this person is a real human and not a troll/spammer". Webs/chains of endorsement could be used to prove that someone ought to be able to interact with you. And for any given active interaction attempt ("react", "friend request", "tag"), there could be an opportunity to mark it as "spam", and accounts with lots of spam could become untrusted, and accounts that endorse lots of spam accounts could become untrusted as well.

In principle you could even implement this in an entirely decentralized way with some public-key crypto, though it might be too expensive in practice.