You can use a fingerprint which consist of geolocation and Useragent. If you use javascript you can enhance the fingerprint with further data like screen resolu

You can use a fingerprint which consist of geolocation and Useragent. If you use javascript you can enhance the fingerprint with further data like screen resolution etc. It's not perfect, since some computers might generate the same fingerprint, e.g. corporate networks where everyone uses the same system, but then again, they are also using the same IP. The fingerprint does not eliminate every attempt of cheating, but the method does not rely on cookies and just changing your ip-adress by reconnecting is also not helping, because you use the geolocation and not the IP. TOR, or just changing Proxies would still work, as well as using different browsers. Another method I can think of are flash cookies. Most users don't know about them. Let users vote as often as they like, but only count them once. In order to make sure they believe their votes count you can display the result personally for them and include their additional votes, so they won't go looking, how they are recognized. And if a user does not know about flashcookies it's pretty much impossible to cheat the system - and the flashcookie remains the same regardless of the browser you use.