I haven’t ever tried it on any of my systems, but I’ve always imagined that using something like using Askimet at the API level could be an option — https://akismet.com/development/ . I seem to recall then also finding free versions of anti-spam APIs out there, but can’t remember any names at the moment.
Mm, I’m looking at stopping the account creation in the first place. The account creation is taking up a good chunk of our “Recent Changes” page, and that bothers me the most.