Same thing happened at our site starting Monday. This ZDBlock thing:
http://www.spambotsecurity.com/zbblock.php seems to have helped a lot, although we did have some problems with members getting blocked when trying to use the the site from their phones.
Also I think that thing might work by completely blocking access from certain countries, which may or may not be something you want to do. (ETA:..although it is configurable, and maybe it doesn't block entire countries by default, I'm not sure).