After a few hours of work, I think we reached the most stable configuration ever. A new module was installed that far surpasses my old checks (which were abandoned). The module works like a charm, and in 12 hours it caught and purged about 200 fake accounts. If we continue at this pace, in a couple of months the database will be completely clean of spammers.
I attach a screenshot of a log where you can get an idea of the attacks we get daily, and how the new module works. You can even see that some people tried to register multiple times from the same IP address, after they realized they were caught. And they were caught again

So, end of spam, guys!