It would actually be easy enough for Walmart to anonymize them, by simply recording the transaction as "$50 Prepaid Debit Card" and not record which particular debit card number went to which customer. Also, if you anonymously acquire a prepaid debit card used for a transaction involved with some nefarious purpose, you still don't get picked up, because it may trace to that transaction, but it doesn't trace to you.
It would actually be easy enough for Walmart to switch to paper debit cards that had the amount of the card printed on the front. When you used that card, the cashier simply gave you lower-denomination of cards (say, a $5 debit card when you paid for a 5 dollar item with a $10 debit card).
Once this practice became pervasive enough, unfortunately the government would have to step in to create rules and regulations as to how all the printing would appear, and to prevent fraud. I suggest they mandate the use of engraved printing plates; green magnetic ink; and heavy cotton rag for the card. Oh, and to certain security features like holograms, watermarks, embedded plastic strips, etc.
My god, the level of convenience we'd enjoy would blow away any other form of paying for goods and services literally overnight.