Charge for votes, like they do on some television "talent" shows, and get spammed all the way to the bank!
Seriously, this is a really tough problem, and someday (maybe soon, if you listen to Ray Kurzweil), computers will do testing to screen out humans. The answers I'm adding to the list have obvious drawbacks, but just for the sake of enumeration: moderation (have humans do the testing), and IP-based tracking (limit the number of votes from a host).
