why not add a filed to user meta once they have voted and just check to see if that specific user can vote?
add to your add vote function this lines:
global $current_user;
get_currentuserinfo();
add_user_meta( $current_user->ID , 'voted', true );
now if a user votes it saves a usermeta filed.
then you can create a simple function to check if a user can vote again:
function has_he_voted($user_id){
$v = get_user_meta( $user_id , 'voted', true );
if ($v = true){
return true;
}else{
return false;
}
}
and to use it and check if the user has voted you just call that function passing a user_id:
if (has_he_voted(12)){
//can't vote again ,you can only vote once
//user has already voted
}else{
//you can vote
//user has never voted before
}