One of the most annoying aspects of the internet is SPAM.  Not the tasty SPAM-musubi type, but unsolicited junk messages sent by robots.  Luckily, we have a tool in the survey system to trick those pesky robots and keep your survey SPAM free.
The Survey Spam Block option will require users to enter a short randomly generated code once they complete the survey to prove that they are human.  To enable this feature, go to your survey and hover your courser over Security on the top menu.  Select Hackers & SPAM from that drop down.  Half way down the page is a check box for Survey Spam Block.  Check this box and save your options.  Your survey should see a drastic reduction in SPAM from here on out.