I have no idea because unless someone is doing this because they are annoyed there seems little reason. I can't imagine anyone on this board actually responding to the spam.
Spammers likely have automated tools telling them how trafficked various forums are, and the cost of spamming lots of sites is probably pretty low. They are not doing careful analyses of the interests of members of various forums.
I suggest that the first message or two of each new member be held for moderation.
Could there be two levels of moderators? Any one of us "junior moderators" could zap obvious spam. The senior moderators would continue to use their judgement about when threads have veered off-topic or become too acrimonious.
