When people attempted to vote multiple times, the count would only increase locally in the spammers browser. Only the first vote was saved in the database. The spamming votes we stored in a cookie on client and added them to the results before rendering the result so he believed he submitted multiple votes.
We had some server side logging as well. Was funny to see in the logs, the number of people that tried to submit multiple votes and to see a few user submitting a lot before realizing the votes didn't get added to the db.
Was very effective and reduced the worst of the spamming significantly.