Here "her" refers to the city of Samaria. This means nothing can stop the enemy army from destroying the people who live there. (See: [[rc://en/ta/man/jit/figs-metaphor]])
Micah uses contagious disease as a metaphor for the army that Yahweh has sent to judge Samaria. Here "it" refers to the "wound," that is, to the army that God will use to punish Samaria. (See: [[rc://en/ta/man/jit/figs-metaphor]])