Absolutely anything that can be exchanged for something of value can be used to buy anything at all. Buy a bag of groceries and swap them for drugs or sex or a gun.
So quit with SNAP altogether and simply hand out boxes of approved food. Then (a) find out the distributors are selling it illegally and (b) the recipients are swapping the good food for other stuff.
This is a fool's errand. It's a feel-good issue for busybodies, analogous to the security theater at airports.