This becomes difficult when you are dealing with distributed systems. Let's use email as an example - I can close my Gmail account, and Google can delete all o

This becomes difficult when you are dealing with distributed systems. Let's use email as an example - I can close my Gmail account, and Google can delete all of my data off of their systems. There is nothing that either Google or I can do, however, to ensure that everyone I ever sent an email to will delete those messages as well. The messages are now on their computers, out of my reach. This becomes more evident when it's not a 1:1 message, but a 1:many message, such as Usenet (or Tavern) Like Gmail, you can delete your account locally, but there's nothing anyone can do that would force other servers to delete the messages they received. Once it's gone, it's gone. Another example where you might see this is XMPP->XMPP. My AIM client can keep a log of your messages, and even if you delete your AIM account, my logs will remain. I can entirely understand where you are coming from, but with some classes of systems, I don't know that it's possible to ensure that your data can be entirely erased.