DePerc:
Instead of buying food, he would just steal it, possibly at the point of a gun. A greengrocer isn't going to put his life at risk by trying to stop someone with a gun from taking a few vegetables. At least most wouldn't. So, if this person gets all that he needs to sustain himself through robbery, how would a DRO system deal with that in a non-violent way? There are no police to coerce him into changing his behaviour, and he has already been ostracized as much as possible. Assuming nobody breaks the NAP, the only way I could think of resolving this issue is through incentives given to this person to rejoin the society and "play nice". But this undermines the whole process of ostracism, if people know that all they need to do to rejoin is threaten people with crime.
Thoughts?
In this respect, I think we have to look to developing technologies like fingerscanning to solve the problem of stealing. If your fingerprint is not in the database, you can't even open the food bin in the supermarket..... or something like that. A supermarket with problems like this ostracized man could hire a security guard to stand at the store exits. And like you said, the only way to non-violently resolve this would be to bribe the outcast to get back in the society, but if there was no security (technology or human) stopping him from stealing then why would this person want back in anyways? Free market problems are solved by prevention, not enforcement.