Can I suggest a deterrent, which might ease other problems too? That is to bar guests from posting directly, but ask them to supply an email address to which a message is sent, and require that they must send the email back to get the post listed. If some people are worried about privacy, assure them that email addresses will be deleted when the reply is received or within 24 hours.

It will certainly deter some guests, but I think not most bona-fide ones.