Just delete them.....
But if that's too much effort, consider a third-party forum product where
you can easily moderate it i.e. members have to sign up for an account
(including confirming that the *actual person* did sign up for an account,
and not just a spam bot trying to get around the system, using that person's
email for whatever reason), and you can set it to approve/deny any messages
posted by members as you see fit.
The FP discussion forum is very basic, but not much good if you're getting
in appropriate postings/spam.
