If you have image verification enabled and the spammers are still signing up, they are real people signing up rather than an individual using a script. This may mean that you could look at the product they are wishing to advertise, then attempt to block all emails related to that or something similar.
What I am trying to say is that, as they are actual people signing up, it is either a single indiviual or hired individuals, meaning that they will most likely be wishing to advertise a single product. If you can find out what this is then you may be able to block it. However, if you continue to delete the spam messages/users then I'm sure that the spammers will simply become bored/realise that their attempts are useless after a while, and will cease to attempt to spam your board.
Good luck.
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