With all the spam Guestbooks have become a thing of the past because unless you are going to moderate them they will fill with thousands of spam and trackback messages.
Once your system fills up with the spam your webspace will be used up and your bandwidth will go way up as others load the huge pages of spam.
Once the search systems see you linking to this spam your page will go down in the ratings as you will be identified with the spammers. Even iof you moderate if the spam stays on your site for more than a day your domain could be damaged by it.
I switched off all my guestbooks and converted to wordpress because it has spam plugins that are mostly automated this still allows users to comment but doesnt take up all my time watching for spam.
Suggest Wordpress with Spankarma and bad behaivor.
Just post a message for your new guestbook and link to that post from your old guestbook link.
It would be nice if someone would write a new guestbook program that could work with the wordpress anti spam plugins but until they do I recomend everyone shutdown your guestbooks and convert to WP.
Having these old guestbooks up is encourging all the guestbook spamers.