The Contact form included with the solution pack has three security measures in place.
1) Honey Pots - These are two hidden form elements that are validated to ensure that they are blank. The idea is that if a Robot is hijacking the form, they will will fill in all of the form elements including the hidden ones. If this happens, the form will fail validation.
2) Security Question - This is a question that only a human filling the form would be able to answer correctly
3) Captcha - Similar to the security question, this is an image that shows random alpha numeric characters that only a human would be able to read and enter correctly.
It is possible that a person is manually exploiting your form, but there isn't a lot that can be about that. you could ban access based on the IP address, but that doesn't work if the IP changes.