When you put a game in your Xbox 360, whether you like it or not, its achievements are stored in a library and add up. If you have 0 points for a game and have no intention to play it in the future, then deleting the game is very easy.
Press the Xbox guide button on your controller and go to Achievements. Scroll down to any game that has 0 achievement points and press the X button, thus deleting the game from your history and increasing your overall completion.
Even if you have just one achievement for a game, you cannot delete it; you have to have 0 points for the game you want to delete. You can check your overall achievement completion on your Xbox.com profile.
