Scientific American reports that efforts are underway to make healthier choices more accessible to people who are on stamps. For example, if participants use their stamps to buy healthy food, additional healthy food will be free of charge.
Structuring the food stamp program differently could help as well. Right now, stamps are given at the beginning of every month. Splitting the distribution up into different weeks could help users from falling into an easy cycle of binging at the beginning of the month, and starving themselves towards the end of the month.
I'm guessing that alternative solutions, like reducing stamps for every pound of body mass over the recommended maximum, are not on the table.