In online voting, you can't get 100% secure, unless you give each user a unique password for voting (and even then it's not fully secure). However you can solve the issue in a very high percentage of the cases, using cheat voting prevention mechanisms.
We offer 3 main mechanisms:
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 most cases, #2 is enough but for an added layer of security, we highly recommend using #2 & #3 together.
In addition, after the vote ends you can export all the vote results to verify there aren't any cheat patterns.