Joomla has the ability to display a Captcha on a form to avoid spammers writing scripts to automatically submit your form. The core Joomla currently includes Recaptcha, which is a popular service provided by google.
To enable Recaptcha on your site, first enable the "Captcha - Recaptcha" plugin on your site.
Make sure you enter your public and private key, which you can get for free at http://www.google.com/recaptcha
In your site's global configuration, set Captcha - ReCaptcha as the default captcha.
Then in your survey category, set use captcha to Yes.
You should now see Recaptcha appearing on your site. If it does not appear, make sure you are using Joomla 3.2.1 or higher, as the URL used by Recaptcha changed and older versions of Joomla need to be patched to make ReCaptcha work http://seepeoplesoftware.com/blog/231-joomla-recaptcha-fix.html