Add a way to restrict your own purchasing. Rather than have the social program give you X money for food and Y for rent, you get BI, and then you can assign X to food and Y for rent /yourself/, so that your card (because it'll probably be a card) rejects purchases that don't fit your self-assigned allotments.

Heck, that might even work for normal people who want a better way to handle their money. Does that exist yet?