You can try akismet. You can try approving all users (which is very inconvenient). But if an actual human is trying to be a pest there isn’t much you can do.
I think there may still be a shadow ban plugin. Looking for somebody to create a shadow ban plugin (I don’t know if they created it or if it’s available). Or you could just keep the user in a group that everyone has muted.
Edit: it does exist: Discourse Shadowban might be a way to get the one bad actor to not realize that what he’s doing to be malicious isn’t having any effect.