Federated systems are one option for this. On one of my sites, the only way people can leave comments is with ActivityPub. They must have a (probably pseudonymous) account on a server to use that, and I hope that most servers have moderation I find acceptable. I can block those that do not.
More sophisticated options for sharing reputation between servers would help here. If, for example five servers I trust block another server as a source of harassment, I'd like to block it as well, automatically.