It's there a way to turn off or get rid of the screen capture?
Not really...but that feature won't work unless if you hold the (arrow back button) and the (power on/off button) in same time and then let them go in the same time too..
And if you don't do that, the screen Capture feature won't work at all.
That feature is off already unless you activated by following what's in above...
So why it is a problem for you?
Maybe he has a skin on top of whatever device he has.
I know with my Samsung tab 2 7.0 I'm always accidentally hitting the screen shot button that is right beside the home and back buttons.
It's actually really irritating because not only does it take a screen shot, but also opens an editor type of menu.