Hi there @Add_97618 welcome to 3Community. Switching off your data once your plan expires is not an available service. I can, however, suggest that you set up a recurring top-up up which will mean that your account is automatically topped up, and this way your plan will automatically renew. If this is something you're interested in please have a look at our blog How to: Schedule a Top-Up.
If this is not an option for you, I know that some customers use calendar reminders. They use these to remind themselves to switch off their mobile data until they top up.
I hope this was helpful if you have more questions please let me know.
