I killed the spammers pretty thoroughly on a phpbb board I ran for my old WoW guild by requiring a question to be answered that spammers wouldn't know the answer to. In that case, it was "who is the guild leader" which is pretty easy to look up on the armory or ask someone in-game. For here, maybe ask what game brought them here? Answers could be Diablo, DIII, Warcraft, WoW, ...any common variants of the games folks play here.
That may not stop it if there's a human who can understand the board names, but it should at least stop the automated captcha breakers.
That may not stop it if there's a human who can understand the board names, but it should at least stop the automated captcha breakers.