I've actually used validation scripts to parse for known SPAM submissions.
I also do the same in my server-side validation (I use ASP), where it redirects the form to my homepage rather than logging the form submission.
I wrote my validation scripts by looking for commonalities on every SPAM submission. For example, the first and last name fields tend to get the same text, so if FirstName = LastName on my form, the form never gets submitted (as mentioned above). Writing these scripts is time consuming and not fool-proof, but it seems to thwart the majority of attacks.