After data bundle is depleted all airtime taken

Vodacom should know that out of bundle rates are not practical at around two rands per megabytes. Every time I buy data bundles at the end of my data bundles the system will only warn me of 5 Mb left when it has already taken all the airtime I have. This should stop. It's *******. The system should warn when there is still enough time to allow switching off of data connectivity. Or better still automatically cease data connection while asking the customer whether to continue into using airtime.