Defeating guestbook spam

Hello Everyone,
Seems I ran into a small issue. I have a guestbook on my site that is being targetted by Viagra spam bots, and other assorted pill pushing meatheads.
I have been trying to figure out an effective way to eliminate the spiders entries, but can’t seem to find a solution.

I was looking into the image box method of verification, but can’t seem to find any literature or a tutorial on how to make that happen. I have also been thinking about adding a password field in the form and have it as a required field, but am lacking in the javascript knowledge to make that happen as well.

If anyone has any other ideas or knows where I can find some literature on how to stop the random but frequent viagra spam, please let me know.

I will be forever in your debt . . well not forever, maybe just the next week or so . .