Microsoft decided to start banning Xbox360 with modified firmwares today.
While it's the same release date as Halo3 beta, it seems that people who didn't download Halo3 also get banned. They probably detect backup discs when you play 'em online (not confirmed yet).
Even people using the new firmwares with disc-jitter added get banned, so it looks like Microsoft found another way to detect it.
Just like on Xbox1 it seems Microsoft bans the console unique ID, not the LIVE user.
Xbox-Scene News: Microsoft Starts Banning 360s on LIVE - MS Confirms