It depends on the contract.
The 'unlimited' plans have contract language that if the customer were to exceed X GB per month, their network connection will be throttled. However they are still able to use the network to get and send data.
The alternative is to either stop when the amount of data send/received hits X or to charge a premium if they go over for the next Y GB and to automatically charge Y for every GB they go over on their plan.
If you think this is bad, try having commercial grade service on the same network that provides residential service for your broadband. I would be pissed except that I can survive the downtime and if necessary use my cell as a router where the penalty for going over my data plan is much than the cost of going with an ISP and trying to get a fiber pulled to the building. (Aren't SOHO's great! :-P )