I don't understand why people share computers like that. It's much easier when everyone has their own accounts, and not everyone can do anything they feel like. Specially now that Ubuntu learns from the users actions, it becomes much more useful to have different users.
Each member of the family should have their own Ubuntu One account and then share folders with the others. Then you can simply leave the box to allow users to change files unchecked. A shared environment like you seem to have, is very difficult to administrate and there is absolutely no reason to do it.