A bit of a tricky situation, to prevent spammers.
A simple approach would be to try a 'question/answer' field that is
'required' (using the field validation).
Something simple like a question, e.g. "What is 4 + 2" and have the user
type in the answer (long hand).
So the example about would be "What is 4 + 2?" the user would type "six".
This way the spam bots (which are not that smart) won't be able to submit
the form with their junk because of the required field, prevents the
autosubmission through the form.
That method is known as "captcha" in 'net terms (you might see a similar
thing if you sign up with a yahoo newsgroup, where you sign up through a
form, and you type in a code which is shown in the form of an image with a
range of distorted characters, as well as an email to "confirm your
registration" type thing.
Maybe you don't want it that complicated, so a simple "what is 4 + 2" and
have the answer typed in is the easiest way. Even making it a random
question/answer might help, but this requires server side programming.
Try it and see if the spam stops.
You might also rename the page instead of using something obvious like
"guestbook.html" or "guest.html", use a jumble of letters like
"asH78K46g2.html" as the guestbook's file name.
