Clean Install MacBook Pro
You can do that by shutting down your computer, then when you turn it on press and hold command+option+R and wait a while (normally about 5 mins)

Once your in Internet Recovery go to disk utility and format your drive. After you have done that go to Install OS X and follow the instructions. This will create an clean install. Once you do that shutdown the computer and then you can give it back to them. Don't do the setup.