You are not allowing Guest posting and you have has no new registrations since I fixed your Board. You do have members who registered from before who are posting. As Brf Says, delete the accounts as the posts appear. If you wish, you may put all Registered users also in the Newly registered users group and then you will have to approve their first (or next) post. If they are spammers, just delete their account. If not, approve the post and remove them from the Newly registered users group. Pretty soon, you will be all caught up.
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