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Suggestions for Spam
It could help a bit if you could, if possible,
Configure your AntiSpam mechanism to require for, the first at least 20 posts, of a new user to fill an alphanumeric code before being able to post.
I know it's not the best solution but it could help until a more solid message filtering mechanism is established.
Consider also, that I have observed that some " newbies" create and leave an account in a dormant state for a couple of days before using it for spam.
That should be a good start...