There is no way to acuratly check who has voted so a cobination of cookies and last voters ID number are used to prevent mutiple votes, but of course like most things on the Internet this can be got round,
There is little that can be done about this eccept change the way it works to store every single persons ID for every poll vote, in every poll, which if say a thousand people voted in a poll this would be allot of ID numbers to store in the database relating to this one poll, if the same where to happen in 10 polls, that would mean over 10,000 ID numbers stored in the database! Which in turn would be allot of overhead in the database making the database size much larger than need be thuse slowing down the forum.
So the solution at the moment, although not perfect and can be got round is the best balance between performance and protection against mutiple votes.