Originally Posted by xkylar
The only thing you can do is to password-protect the entire device afaik.
Yep, this is the only way. Protect the entire device....and tell your friends to keep their grubby paws off of it.
If your 'friends' enter the password wrong 10 times, all data will be deleted from the device, so if you think they are the type to do that, make sure you do frequent backups.