Any poll that uses cookies can be compromised. Any they can't do it by IP since often companies only have a single public IP for hundreds, thousands or even ten of thousands of users. With ANY form of electronic voting, there is NO way to prevent fraud 100%.
Only hand-written/signed paper ballots would be secure. With surveilance footage of the person writing it.
I think that this poll is a good indication, but it isn't 100%.