Simple fix for the time being:
1. Set a passcode number.
2. Set a double-click of the Home button to take you to the Home screen.
(Settings -> General -> Home Button (scroll down to see that) and select "Home")
If the iPhone is locked (happens whenever you turn it off or don't use it for your specified time). All the 'new' user can do is get to
either[/]the Emergency Call button/screen OR the Unlock Screen.