I run image verification....aggressive spammers actually use real people to get thru those....figger yer payin some barefoot kid in India a penny everytime they type in those characters or reply to the activation email.
I use image verification for all registrations and require the register reply to an activation email.....they still show up. I've done some mass email domain bans as well as some IP bans, but since my site has a worldwide userbase, the risk is there for blocking someone legitimate.