What you can get is RESET that means you lose all your credits, all your armor, and your rank.
You keep your commendations (thats to make it harder for you to boost as you gain less Cr the higher your commendations)
Also you will get credit locked for a period of time, that means that you cannot gain Credits for a while. (TEMPORARY!)
But you CANNOT GET BANNED!!!! (banned meaning not allowed to play Halo Reach online anymore)
Any type of Network manipulation (unconecting cord, signing with an offline profile to confuse the system, putting your modem on standby etc etc CAN alert bungie and they can reset you)
If you replay Checkpoints, or Play some type of game a lot you WILL NOT GET RESET OR BANNED.