unless you change the format so that your guest-book is pre-moderated. That means that each entry is submitted to you off-line, and you check it before uploading it.
Frankly that's a pain, and makes the whole idea of a guest-book unworkable. You can carry on as you are, and hope that the phantom spammer goes away, but once they get your address-book location they tend to keep on and on. Delete the entries and they will almost certainly reappear - I've had the same problems with some clients' open discussion forums.