Was reading on the XDA site and came across this:
FaceLock Face Recognition App Locking for Gingerbread
I've come across situations on this forum where Galaxy owners have let other people or their kids play with their phone, when the device is handed back something doesn't work, settings are changed or the phone is locked.
The above link is to an app which allows you to lock: Settings, Play Store, Task Manager, and one application.
Hope this helps some of you lot and thanks to XDA.
Requires a front facing camera and Android 2.3 or above.