Welcome to the community @Partacidcharm!
As mentioned on this thread, we don't block data usage. Some devices however offer the possibility to restrict the data when you reach a certain amount, but this is only available in the settings of the phone itself.
A block is added on the account when the user reaches $50 of over usage but this can be lifted if the user accepts the charges and agrees to continue being charged.
Don't forget, since you're the account holder you're able to monitor the usage in many different ways.
Have you thought of doing a Transfer of responsibility?
That’s really unfortunate. I know other companies will put a data block on. He will not pass a credit check so will be unable to separate the account. I guess I will just have to cancel the account as I do not want to be stuck paying overage charges.