Revocable File Sharing?
pascalchristian: the file does not need to be erased, the file can be encrypted with a key which is hosted on a secure server. after a user uses a key for sometime, it would expire and they would need to request a new key from the server. however you can set the server to stop issuing keys, and hence the file would never be opened again. this is similar to shareware distribution, it has been done for ages i guess. think steam.