Potential method for erasing your high scores post-NXE
Credit goes to Lightning 009 from TrueAchievements.com. Personally I couldn't get this to work because I would sign in too fast but I'm interested to find out if anyone else can get this to work...
First, you'll need a secondary profile. Setup your 360 to automatically start the game in the game disc (at startup) and automatically sign in to your secondary (boosting) profile. Now with Civil War disc in the tray, and both settings applied, turn off your 360. Now turn the 360 back on (probably with the controller because you'll need that ready to go in just a second). Here is the key, you'll need to press the guide button and be ready to sign in to your main profile before the secondary profile signs in, and just soon as the little indicator pops up showing that your secondary profile has signed in you'll want to sign in with your main profile. This should erase all of your mission progress and high scores.
As I stated before, my secondary profile was signing in before I could even get the guide menu open, but its worth a shot. Perhaps my internet was too fast? Be sure to post results if you make an attempt(s).