Remove admin account
He could format THEN REINSTALL the OS and power off before the initial setup. 

Also there is a way to force setup assistant to run without reinstalling the system (some unix knowledge required and yes, this works in Lion):

Boot to single user mode

mount -uw
rm -rf /Library/Preferences/
rm -rf /Users/username
rm /var/db/dslocal/nodes/Default/users/username.plist
rm -rf /var/db/.AppleSetupDone
exit


(Note: replace username with whatever account short name you’ve created)