One of the main concepts in the Green notion of tragedy is that of "hamartia". It is derived from the verb "hamartein" which means to miss a target with a bow or a spear. An intrinsic part of this concept is inevitability. Once and arrow or a spear as been released it cannot be recalled but continues in the direction in which it is pointed.
Creon's decision to wall up Antigone in the cave at the end of Scene VI is such a moment. He decrees:
I shall take her to a place completely
devoid of human life and hide her, living,
in a rocky cavern.
When he reverses his decision and removes the rock from the mouth of the cave, having been convinced that he made a mistake, it is indeed too late as she has handed herself. We do not see the actual hanging, which occurs between the end of Scene 6 and the beginning of Scene 7.