This is interesting. What if all homeless people in SF were issued a cash card. The balance of the card would be controlled remotely and it could be (de)activated at any time. People who want to help the homeless could pool money together and the funds could be put onto the cards at the point of need. Perhaps retailers would be able to request the balance on behalf of the purchaser somehow - phone, web, etc. If people stopped giving cash to homeless, a system such as this could restrict the flow of money to drug dealers and liquor stores.