said by Dampier :

Residents in some cities in Texas get called by "security" when they exceed 40GB of usage in one week. Their service is suspended until they listen to a lecture about excessive use and then it is restored.

It's my opinion these decisions are made within each division and there is no national policy at this time. If you live in a "live and let live" area, no calls. If you don't, you should expect service interruptions and lectures. I don't know of anyone banned from the service however.