If you are allowing Everyone to vote - then votes are tracked by IP. If you allow only registered users to vote, then it's tracked by username. AOL users don't ever have reliable IP info - they have a different IP on every page they visit as AOL (and a few other of the larger services) use proxy servers.
So it's tough to do anything by IP with AOL users. For the most accuracy here - in your config file, set it up that only registered users can vote, then it'll track votes by username and thus only allow one vote per user and more accurately show if they have already voted.
