Also, server plugins can prevent players from accessing blocks, so, yeah.
But this is ComputerCraft, a mod that is designed to mostly be an open system for players to do whatever they please. If you're worried, go and write your own system that encrypts files on disk with AES encryption and needs the password on boot to access files, as well as requires the password after it goes to sleep to unlock the computer. (It is completely possible, I've done it before.)
EDIT: This upgrade would handle the transformation Java-side, and wouldn't require the user to label their computer (although they should), to make it very straightforward. Also, the reboot issue, ever heard of a startup file and saving your state?
Edited by awsmazinggenius, 19 April 2014 - 10:27 PM.