There is no way to stop cheating in polls. Borgs' method is the best.
Here is a list of ways to protect cheating and ways of countering them.
Proof that you can always cheat in polls.
1------
Not feasible
Cheat Protection: Log each user that voted in the database.
Counter: Make new accounts and vote.
2------
Not feasible
Cheat Protection: Log every user's IP.
Counter: Use proxy servers or windows gateways.
3------
Normal way
Cheat Protection: Set a cookie.
Counter: Delete cookie.
4------
Borgs' way
Cheat Protection: Set a cookie & Log last voters IP
Counter: Delete cookie and wait until someone else votes or use a proxy server/windows gateway
There are more ways but they are all a variation of one or a combination.
Methods 1 and 2 are not feasible because of database limitation.