As I said earlier, I've seen plenty who were able to peel off $20's to buy a carton of smokes, while using the SNAP card to pay for "food" at a gas station convenience store.
If you have the money to smoke, you shouldn't be getting financial assistance for your food. Short of testing welfare recipients for tobacco or drugs, that's also impossible to stop, unless you somehow have the retailers get in the middle as gatekeeper (i.e. refusing sale of the tobacco after selling them something on SNAP).