rogerd - 8:02 pm on Jul 29, 2013 (gmt 0)
Some kind of human verification is useful, but incomprehensible captchas are not the best. I've struggled with them myself, and it's easy to see how an unmotivated user might try once and move on to another site.
I installed a vBulletin plugin that uses a multi-layered defense including Akismet; it didn't stop all spam registrations, but minimized them. The only strategy I turned off was username recognition. So many common usernames have been used by spammers that legitimate users attempting to register with those same names were getting blocked.