Blaze. I get this same problem on my forums, but I found a better way to deal with it.
Make the registrations activate by admin, and add a new question on the registration screen - "Please post a link to the MFF website."
Works brilliantly for me, because the spambots usually post some stupid website in that box. I just delete those ones, and accept the users who post the right website.
P.S
Obviously on my forum I don't ask them to link here...