There should be another option with moderated guestbook postings. Currently the options for the moderator are accept or reject. But there should be another one where you could reject and ban the IP address. The only way around this at the moment is to accept the message (which exposes it in public), then quickly go and delete the message and ban the IP at that point.
Also, I had a problem where someone was copying and pasting text from a WWW site which had tabulated columns in it. Because various fields in the Guestbook are separated by tabs in the file which holds all of the text, this caused very strange things to happen like suddenly adding a comment from me (which was followed by some numbers from the tabulated fields), starting up a new comment (Submitted by), and so forth. Might be a good idea to disallow the use of tabs or just cause them to be stripped out if they are used.