I have a similar problem.
Brand new forum, tons of users are registering, most of them with gmail addresses (so can't ban the domain), IP addresses belonging to networks in Pakistan, Poland, Russia, etc.
They haven't posted yet, but obviously it's a ticking timebomb.
I have CAPTCHA enabled, and also have Project Honeypot integrated.
Only other options I have are Q&A (but that means I have to disable CAPTCHA), or enable CloudFlare, which can be a pain as well.