Multiple voting on a poll.

[Rask]

I have a poll feature I made awhile ago. Usually, it's hosted some inconsequential polls so all I've used is things preventing people from voting more than once from the same IP, and sending a cookie when you vote which doesn't allow you to vote again.

 

This is obviously fairly abusable. I'm going to be hosting a contest with a reward pretty soon (nothing substantial, but a reward in a game) which I want to use the poll feature for.

 

So what I'd like to do is figure out a way to make my poll less abusable. I haven't been able to come up with anything, so far. I know nothing will be bullet proof, but importantly, I don't want to have to setup and sort of login/authentication feature. Is there anything I can do?