"Well, if the Mudcat 'Guest' system is tying in real 'Guest' posts, with false ones, surely it needs to be changed as fast as is possible."

How? Guest posts allow you to post without signing in, and without a cookie being set. So someone (say, you) can log in as Guest Lizzie Cornish, but so can anyone else. Doing a search on the name Guest Lizzie Cornish will pull up all instances of that Guest id being used. That's a completely reasonable aspect of the Guest posting facility. Of course, it doesn't stop anyone, anywhere, using your Guest log-in to impersonate you. The solution is to always log in, so that it's obvious that any guest post isn't really you; or for the Guest post option to be removed by Mudcat altogether.