We offer 3 main mechanisms to prevent repeat voting:
1) Blocking repeat voting using the browser - this is not very strong, as users can just delete the browser "memory" to circumvent this mechanism.
2) Blocking repeat voting using IP addresses - this is much stronger and more than enough for most of the cases, however, a more sophisticated hacker can overcome this by writing a bot that switches IP's automatically - not easy, but possible.
3) Blocking repeat voting using social profiles - in this case, you can only vote using a social profile (Facebook, Google, Twitter).
After the vote ends you can also export all the vote submissions to verify there aren't any cheat patterns (e.g. irregular submission times, bursts of votes to a specific side, etc).