This site is getting at least 25 spam attacks a day, and two methods of prevention are used:
“Stop spammer registrations” from Keith Graham, http://www.blogseye.com/i-make-plugins/stop-spammer-registrations-plugin/
which checks known spammer databases, and many other variables. If this stops you, and it does appear to go through some periods of great enthusiasm, then please leave a message in the box that comes up and we will join you up manually, and “white list” your email and ip address.

“User spam remover” from Joel Hardi deletes any allowed registration that has not been used for a couple of days. Spammers will often register on thousands of sites before leaving any spam at all, to avoid Keith Graham’s prevention, and then return a week or more later and blitz all the sites with spam. This is why we ask you to leave some comment or forum post as soon as possible after registering.