elementary OS designed parental control, but in a bit of an opposite way as you've suggested. An administrator can create a new standard account, and restrict its access to the computer. For example, the standard user can log in between 5 PM and 10 PM, can have access to some of the applications restricted (as in, he can't run them) and he can be banned from accessing certain websites (blacklist-only options at the moment).

Not really what you're looking for, but built exactly in a way you want it: available out-of-the-box, tucked into system settings, there in case you need it. Could be of some inspiration to Ubuntu guys.