I have a suggestion for anyone using phpBB, MyBB, or Vbulliten boards. There is a website that lists spammers by email, IP, and username. (stopforumspam.com). They have plugins/mods for all three of these forum softwares available via the link below.
http://www.stopforumspam.com/downloads/However, for the sake of actually getting members, do NOT ban the entire list of spammer domains! That list includes very popular email services including yahoo, msn/hotmail, gmail, and many more. The TOP domain for spammers seems to be gmail; which is actually the email service I use most of the time because of some of its features (even though I have email with my domains, I prefer using gmail). Ban/disallow specific emails, IPs, etc. if you wish (especially if you can't install the mod/plugin or are blocking the IPs of spambots on the server level instead of via the forums ACP).