Does anyone know if there is a way to disable casual data on a spark prepay phone?

A friend says he had a 100mb data plan but after the 100mb was used it started charging excess data to his balance when he would have preferred data to just be blocked.

He also made the mistake of replacing the samsung firmware on his phone that has a nice disable data button to some cyanogenmod which has the data on/off button through three levels of menus, but thats beside the point.

Can he set his account to dis-allow casual data?