Re: Tamperproof Install
Log in with the "guest" account - all the Gnome settings will be at default, the guest account is extremely unprivileged so they can't change settings, and the contents of the guest home directory is erased on logout.
Set the "login window" settings so that, after 10 seconds at the login screen, it will log into the guest account.
I try to treat the cause, not the symptom. I avoid the terminal in instructions, unless it's easier or necessary. My instructions will work within the Ubuntu system, instead of breaking or subverting it. Those are the three guarantees to the helpee.