Just went and checked (out of curiosity) on one of the sites I mod, and there are PAGES and PAGES (well over 15) of "moderator action log" that are nothing but :
Banned IP for reason “Found in the Stop Forum Spam database”
(except for 3 new IPs I banned over the past 3 weeks, and the corresponding deletion of the SPAM msgs).
I *highly* recommend banning by IP, and IP subnet. Otherwise (the mod just bans the username) they just create a new username from the same IP (or IP subnet).
You are kinda screwed if the subnet (owner of IP adr range) is HUGe and in the US (such as a cell carrier, or a cable provider - like Sprint or Comcast) because you can't really block those ranges or you block a bunch of valid people who might get assigned that IP.
Your forum software *might* allow a (say) week-long ban only, so that you can block A.B.C.* for 7 days, enough to maybe get the spammer to move on, but then allow others back on.
Again, good luck.