There is no "captcha" for WF forms. I went through the forum and found a decent workaround. I put a button over the "submit" button and had it disappear when clicked on, exposing the "submit" button.
I don't know if it does any good but it creates a small sense of security.
I think it would be even better if the interaction, in addition to making the top button disappear, would add some data into a required field that isn't visible. The user wouldn't be able to "submit" without that required field having some data.
I'm not sure this provides any great security, but it does provide a little more of a secure feeling.
Is it possible to add something into a field through the interaction?