spam posting on guest account even banned posting

We appear to have a problem whereby the guest account ("Guests") has posted several spam messages across the server.

The settings for the guest account is to allow access (view) but no posting and yet this has not been enough.

When I try to edit the permissions for that account to check the permissions etc it won't let me from the admin - I select the user and click Go and the page refreshes and doesn't show me the user's permissions.

All other users work as required.

Any ideas anyone?

thanks!