Welcome to WebmasterWorld Guest from 22.214.171.124
[edited by: dreamcatcher at 6:15 pm (utc) on Sep. 10, 2008]
[edit reason] No urls please! [/edit]
Check user-agents - see if you get multiple votes from the same unusual agents.
There's a few other things you could do. I guess it depends whether you're trying to stop a user clicking a few times, a bot trying to vote 1000s of times, or both.