CAPTCHA is probably the best solution to prevent bots. You can add the CAPTCHA field to your form and a field where someone enters the characters displayed. Then do LIKE value comparison from the SESSION variable created by the captcha compared to the form element you place.
The Sever Variable to validate is usually the form element you choose from the bindings. If you do your best to get started and then attach your page to a reply I can comment on what is left or what is wrong.