Uhm... really there would be no way to fully ensure that the game would be rated once the user completes the game. If a user just skims from game to game, rating them without actually playing them, well that's their fault, and it will eventually become obvious. However there isn't anything that can be done to stop it, and unfortunately if it does have an effect on the games rating, well, again nothing can be done. Though, if the game is popular enough there should be enough ratings to overpower the nonsense votes to get it to the correct rating.
