Hello,
YES! well spamming a guest book is a very very common experience I would suggest. I too could have gone down the guest book route you've taken but I found a nice solution to my needs I don't know if its of any interest to you, see here:-
http://www.a-free-guestbook.com/
Sadly I do still get the odd bit of annoying spam but I can login to my free account & remove it within a few moments or a few mouse clicks. (i.e. message management).
**I think no matter what kind of guest book you create spam is going to be an annoying fact of life :-(
Ivan