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If you didn't already know this, you can allow the user to vote, and then send them a cookie. If they try to vote again (before the cookie has expired) you can block their vote by checking their machine for the presence of the cookie.
But as it's been noted here by Roger, cookies can be deleted. And the user could also vote from a different machine to get around this limitation if he or she didn't know how to delete the cookie (or even knew what a cookie is).
Still, I think using a cookie is your best bet here. Most computer users are not all that savvy about cookies. Most would not even know it's a cookie that is blocking them from voting more than once. And most users, even if they knew it was a cookie doing this, would not even know where to look for it on their machine so that they could delete it.
You are going to have some cheating no matter what you do. Perhaps if this is that important to your survey you will need to do some statistical analysis to determine what percentage of people cheat on such surveys, and then note your results on that basis. Also know as ye old "This survey has an accuracy of plus or minus "X" percentage points."[/FONT]
