nothing more can be said about this You are not allowed to stay up to see that movie and that's that.
Usage notes: used to show you are very determined
that's that
Also, that takes care of that. There's no more to be said or done; the matter is finished, the issue is settled. For example, Dad's not buying you a television set, and that's that, or We've paid all we owe, and that takes care of that. [Early 1800s]