1. you didnt reply to my comment
2. Ask yourself that question, hwo do your banks handle customers who don't abide to repay?
Assuming your answer is 'throw em in jail', Islam is more practical than that, if the person who default's has the money but isn't paying, then the matter is taken to court and the judge will make the appropriate decision, either the bank agrees to extend his contract, or he is forced to pay, i cant answer whether that's deemed to be theft but if it is then the punishment is applied.
But obviously the bank won't go out lending money to anyone and anyhow, similar to the way usury banks insure the person with the loan has a security.
If the person who needs the loan is poor, and the private bank can't come to any agreement with him/her, then the treasury is there to help, read into Umar's life as a khalifah to see examples of that. (Yes Muslims did have social welfare systems and a treasury while the europeans where trying to figure out whether the woman was a human or not)