SpamAssassin may be useful for e-mail filtering, however, it sounds as though you need a human verification solution to validate form posts.
There are a slew of useful solutions out there, however,
ReCAPTCHA is the easiest one to implement if you are not a developer.
One caution: users who have disabled off-site images may not see the ReCAPTCHA form. Be sure to include an instruction for visitors who have disabled images or are using ad-blocking software on the same page as your form, or as part of an error message displayed when your form is submitted without a valid ReCAPTCHA string.