We had a rash of spammers that would spoof IP addresses, meaning that blocking "their" IP would only be blocking innocent users. They would create new usernames for each spam message, so a username ban didn't work, either. The only common link seemed to be a GMail or Hotmail e-mail address, so it was either ban those or deal with several spammers a day.
If you PM me your banned e-mail address, I can change it in the system for you.