On any protection, have her crawl. The easiest way to bypass and difficult to close it is services like www.wmmail.ru
. Pay peanuts and get tens of thousands of votes from real people with different ip's, browsers, etc as this is real people.
So before thinking about some protection, will have to find them all and monitor tasks on a link to Your site. But many make the order so that the link was on another website and the link from the task. So it's bloody impossible.
Does that make the vote mega complex (enter passport data, etc.), but then real people, nichrome will not participate.
Another option if votes are expected sooooo much more than people in such systems, then there is a chance.
Conclusion, 100% protection, make elementary cover that would have the same ip could not vote n times in x minutes + a recaptcha. It is quite simple from the robot. The rest can't win.
PS: Russian hackers President to the United States has chosen, what can I say now )))