If you were to do this on the client side with javascript it would be pretty simple - however, client side scripts can be circumvented and manipulated, so server side is the best but that requires that the form is submitted and processed. If you want a quick and dirty client side script, I found this on google: stackoverflow.com/questions/3279628/checking-the-form-field-values-before-submitting-that-page that does a good job showing you how to validate a form. With this method you wouldn't need to clear the captcha form for them to resubmit.