Voting Polls and preventing multiple votes

Hi there I am mocking up a quick voting poll system, however I would like to put hooks in place to prevent users posting more than once, voting bots etc. Is there a way to prevent them, obviously sessions, cookies, host ips cant be used as they can be removed, and especially with host ips , ppl are usually behind a proxy ip that doesnt forwarding the referring ip. Let me know if anyone has cooked up something like this thanks. I have the voting mechanism and result displaying fine, just the special checks to prevent the form being displayed.