Found it, and people have confirmed it works.
I'm sure MS will fix it eventually.
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1. Load up the game you want deleted
2. Exit game once it has loaded up (make sure it shows as your most recent game).
3. Load up any game that you have 0g for.
4. Exit that game (once again make sure that game now shows as your most recent game). On your recent games it should now read 0g Game first and the game you want deleted should be beneath that.
5. Now, go to delete the 0g game history. If you notice that as you press "Yes" at the bottom of the screen it still says "Delete Game History". As you press "Yes", quickly press DOWN, X, UP, X. If done correctly, you should have on your screen something like "Are you sure you want to delete your games history?". Select "Yes".
6. All achievement for the game you want deleted should now be removed from your gamerscore.
Obviously care should be taken not to remove any game you don't want to remove gamerscore for.