Since we moved to the new forum software in late 2016, I've had a mostly automated system in place that tries to validate new account registrations. The email/originating ip/some other characteristics are checked against a database of known/suspected spammers and if matched then the registration put on hold until an admin can review details. This has been catching >95% of offenders since then, with almost no false positives. The spammer database gets updated by other forums owners continuously, but it does appear that there's a new source within the past week or two. It's possible they may subside, or we can consider stronger measures.
Some available options include putting new registrations on manual post moderation status until they've posted a certain number of times, or imposing a delay between registration and first post, but new people can find these a bit discouraging.
Happy to hear opinions and additional ideas from members!