[yorwba]

> I can't see a way to prevent users from creating new identities / key pairs for themselves whenever the old one has been banned.

You could prevent banned users from returning with a new identity by disallowing the creation of new identities. E.g. many Mastodon instances disable their signup pages and new users can only be added by the admins.

If you don't want to put restrictions on new identities, you could still treat them as suspect by default. E.g. apply a kind of rate limiting where content created by new users is shown at most once per day and the limit rises slowly as the user's content is viewed more and more without requiring moderation. (This is a half-baked idea I had just now, so I'm sure there are many drawbacks. But it might be worth a shot.)