I think every moderator on every platform is struggling with this issue, and no one has succeeded so far, so it doesn’t seem that easy.
I think a simple solution (and one that eventually every content platform will have to adopt) is to allow users to tag AI-generated spam. I think that a few years from now this feature should be the norm, like existing basic features on forums such as upvote, downvote, favorites, hide, etc. I know this will require much more development effort than simply blocking new accounts from posting at all. But on the other hand, you can’t block new accounts forever.
It’s going to end up like the flag button. Disagree with opinion in the post -> tag as AI-generated spam. Not that I disagree with the idea necessarily, probably a few safeguards around it like only certain users can tag and pattern over time would fix it.