From other forums I've joined before - members are registered people logged in. Guests are people reading who have not registered or logged in and search robots are the likes of Google / Yahoo reading the site to index it and make it appear in search results ( a good thing
A further entry has now been made on that message - which has then generated an email to me. [Note that each is at about the same time of day]
I presume this is someone hacking this system and that therefore this is something you will have to handle.