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I don't know how much spam is added by robots and how much by humans, but I have seen some guestbooks that have been hit really hard.
A protection against the robots is to use a visual security code (the random number sequence presented as an image that you have to read and enter to make a post). I don't know about a guestbook, but I believe there is a login mod that uses this technique?
If you want to allow anonymous comments (which I agree is the idea of a guestbook) cleaning out the guestbook just has to become a regular admin task.
A thread is for life. Not just for Christmas.