It is called a honey pot. It is easy to add. Just use regular expression validation to validate that it is blank. However, spam bots are pretty good about avoiding those these days as well, so it might not work. You might want to consider using our "simple question" validation, which is more reliable than CAPTCHA or honey pot. Google reCAPTCHA is probably the best solution. They are constantly making updates to counter spammer's attempts to get through.