We've already seen some cases of instances get defederated because they hostrd several spam accounts and they didn't have any safeguards to prevent spammers from creating a new account when a current one got banned.

Look at email. Effectively, a few major instances run email and dictate rules on interfacing with those accounts as a way of controlling spam. If you don't follow their rules, your entire server gets blacklisted. I don't see how any federated system won't end up there.