Spam bots are made to be annoying. No matter how many times you ban one they can always make a new account and resume spamming. I've seen two other forums fall heavily victim to such attacks, one of which died because of it.
As for IP bans? I've seen more evidence of those failing (and there are guides on line as to how one goes about changing their IP) and innocents may get caught in the crossfire if I'm understanding what this one FAQ is saying about coffee houses.
