How to compeletey erase memory off iphone 5 that i found?

I found an iphone 5 at my group home, th girl left and never returned. When I tried to restore it, it asked for the apple id she had. How do I completely restore it

Added (1). I've already restored once completely and now when it came to activating it with an apple id. It won't let me log in with my apple id. Is their anyway to completely reset it

Added (2). I want to remove the apple account but I can't. And if your just going to lecture me to take it back. **** off I didn't ask for all of that. This girl stolen more than half of my belongings and my sons stuff so I have no remorse if she really wanted the iphone she would've came back by now and its been 2 months

1. Go to Settings
2. Go to General
3. Scroll all the way down
4. Click Reset