I still think it might have been a techical problem somewhere in systems of the webhosting company, for in the morning the entire database connection was unavailable. That came back by itself after a few hours, so I assumed the tech guys at webhosting did what they could. After that a few database tables were corrupt, and I ran the database system's internal repair command on them.
So, yeah, it might or might not be related to spam bot activity. In any case, I don't know if we could do more to prevent spam accounts from registering without making it too unconvenient for real users to register. I think at the moment we have it so that each new user has to answer two custom questions - I have no idea how the bots bypass that, as I'd guess that even with a brute force attack getting two random words correct might not happen so often.