When someone is a thief it is not a matter of religion but a matter of education.
And may I just add that it's also a matter of being human.
You don't have to use biblical or religious verses just to convince an atheist that stealing is wrong. Yes, stealing is against one of the 10 commandments. But even without knowledge about the 10 commandments of God, one would know that taking something from others without their permission is wrong. How? Just by being human. Like, how would you feel if someone stole from you, right? Just by simply putting oneself in somebody else's shoes would allow one to think about his actions first.
Point is, regardless of religion, regardless of what one believes in, stealing is wrong.