There is no automatic shut off for data when you reach your cap that I've ever heard of. Not on any platform. The only thing I can think of is to get alerts from Verizon for data usage and then manually turn off cellular date (and rely on wifi) when you're close to your cap. Remember; Verizon's data usage on the MyVerizon website is only an estimate. Maybe turn on your usage alert for 90% and then turn off cellular data.