I had the same thing on my forum. Mine was Duh. At first I didn't realize it was spam and I checked my member list and found another user that was waiting for verification. I set it up then told the angry user. Then something clicked in my head and I did a search on the user name and ip. I banned the user as soon as I found out it was spam. I banned both accounts.
Same thing here, AngryUserKer with IP address corresponding to 1 inactive user who registered several weeks ago. Then a 3rd user with same IP registered and posted itubesglobal spam. Eventually banned the IP address.
i've created a custom addon, which is quering the new posts from new users in google
IMO there's NO BETTER anti spam protection
if there are more then 5 results with the exact same post it's definitly spam and gets into the moderation log...
and the biggest pro is, that there's no problem with privacy, because you're not sending any user data (username,usermail,ip) to an 3rd party page/database