There is nowhere specified on the way the company will handle this. In most of the cases, if you have a contract, the speed is just lowered until the subscription is renewed again on the next month as it's specified in the contract. There is also the possibility that when you reach the limit to be forbidden to get access until you renew it or buy more data. It's all on the developers.
Use the following link to obtain additional information: https://www.theice.com/contact