You might enjoy reading Greg Young's https://leanpub.com/esversioning https://leanpub.com/esversioning, which covers this topic. It covers several strategies,

You might enjoy reading Greg Young's https://leanpub.com/esversioning https://leanpub.com/esversioning, which covers this topic. It covers several strategies, three of which are: * Encrypt it and then throw away the key to forget it * Store private data outside the event with the event just pointing to it * Delete events (on systems that support this)