If you are willing to make the users work a little more, you could use a scheme in which a random key is given to the user before he/she can vote, and has to be entered for the vote to be registered. Then you store the random key for a certain period of time, during which any votes that come with the same key are rejected.

The way I have seen this done (here) is that you have to type in your email address, the key is emailed to you, and then you enter it when you vote. This is a form of registration, but it's not permanent and only an email address is needed.

--ZZamboni