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thats always a good option,that and posting a message to new users asking them NOT to spam the boards, as its rude, unappreciated and a violation of private property. You open your boards to decent people, who respect your property, the site is not open to those that spam.
Appoint a team of Moderators that can delete and ban spammers as soon as they post, really, declare war on them, they will soon leave you alone and try elsewhere...
but having the guest account open is a bad idea....
:)