The problem is selectivity. if they stop selling to new customers then they can be sued for discrimination and what not.
What they have done is expanded their coverage area without upgrading their backbone, or creating a new one. this requires them to do one of the following:
1) impose usage caps
3) update modem software to automatically disconnect when not in use like Bellsouth
4)Take reserve funds and upgrade the back bone, though the funds are usually used to expand area and one of the former is imposed.
5) the best decision in the long run, is to stop expanding and use profits from existing area to upgrade the backbone to handle 25% more then what is currently on it and then begin expanding again.