Q: Is there a way to add a human verification feature to the Prospect Widget to help prevent submission spam?
A: Human verification is also referred to as the CAPTCHA. We have done one better, we use a honeypot or reverse human verification system on our form to attempt to detect robots.
A honeypot is not the same thing as a CAPTCHA. In fact, it's the opposite. There is a hidden field that robots/spambots fill out that humans don't. If the field isn't filled out, then we know the submitter of the form is a human. It has all the benefits of a CAPTCHA, except that your clients don't have to try to fill in annoying mis-shapen letters.
Neither are perfect, but it has been working pretty well for our clients. You can read more about the philosophy behind honeypots here: http://en.wikipedia.org/wiki/Honeypot_(computing)