(idiomatic) There is nothing more to say or to do concerning the matter.
Example: It is quite simple. You're grounded for a week and that's that!
1872, Samuel Butler, Erewhon, ch. 6:
They gave a little frightened cry and ran off as hard as they could. "So that's that," said I to myself, as I watched them scampering. I knew that I had better stay where I was.
1908, F. Marion Crawford, The Primadonna, ch. 17:
The jury is satisfied and the verdict is that you needn't fuss. So that's that, and let's talk about something else.