Most of the spammers come from a few particular countries. There are ways and means of weeding these out. Drop me a line if you need some info.
I'm all ears!
We block IP address ranges at our firewall, known spam-originating IP addresses, email addresses and hostnames at the application level via a third-party spam tracking service, and have extensive pattern matching of email addresses, hostnames and IP addresses in our our ban lists which are triggered at the point of registration. Between them these measures prevent hundreds of registrations per day, but obviously some are still getting through.
If there's something else we could be doing please post it, all contributions welcome.