If you are really worried about people faking the votes you could add the following code to the lvlup page but it might stop people using internet providers such as aol from being able to vote. Guess it just depends which is more important honest voting or traffic. I have this same problem with a game I own and people trying to cheat the referral system using proxy servers and this code will stop them.
if( @fsockopen( $_SERVER['REMOTE_ADDR'], 80, $errstr, $errno, 1 ) )
{
die("Proxy Detected You can not use this page via a proxy go to <a href='http://www.yoursite.com'>Your Site</a> to sign up.");
}