We offer 3 main mechanisms to prevent repeat voting:
1) Blocking repeat voting using cookies - this is not very strong, as users can just delete cookies.
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 over come this by writing a bot that switches IP's automatically - not easy, but possible.
3) Blocking using social profiles - in this case we allow voting only if you use a social profile (Facebook, Google, Twitter) for doing that.
In addition, after the vote ends you can export all the vote results to verify there aren't any cheat patterns (e.g. irregular submission times, etc).
Blocking using IP addresses and based on social profiles is available starting on the Starters Plan. See here to upgrade.