This is a very poor response from a service provider.
I have this line for my daughter and she regularly exceeds the data limit. I would expect that once she reaches the max, no more data should be available with the CAP. But the Calls and Text should be available. That is how O2 and Three work. With Virgin Mobile, the phone becomes a brick. Also, even with the CAP, I always get a bill in excess of the CAP, because "the Bill is calculated 15 days after the Monthly rolling period and the system is unaware that the account has exceeded the CAP". I cant wait to cancel the contract.