First off, you talk to them and tell them where they went wrong in a very descriptive manner. So they can understand each and every detail. The parts where they went wrong and why it was wrong. If they still do not understand, and you think it's worth your time to make him/her understand, then you try harder.

One thing you have to always ALWAYS, keep in mind, is that you cannot change a person's behaviour or thinking pattern, but you can definitely change the way you react to it. If the person does not admit to their fault, then there is nothing you can do about it, because you cannot force your thinking onto someone else. If you know the person is guilty, and even after trials over trials of making understand, they do not get it.. it's time for YOU to let it go.

You have to let it go. There are many many people out there who will wrong you to a point where it will destroy your soul and they won't feel an ounce of guilt. And it will anger you and depress you, but at the end of the day… Thy are just not worth your precious time. Use the time to do something more useful, like eating nachos.