[@Cleopatra] I mean it as directly attacking the soul.

The shell magical girls become is still controlled by the soul, and should (correct me if I'm wrong, which I likely am) still need normal things to survive (food, water, air). If you were to, say, shut down the brain by directly manipulating the soul, then the person would stop breathing, their heart would stop, and they would likely die. 

We know that magical girls bleed, because Homura is shown to bleed in Rebellion. Therefore, stopping blood from pumping should have an effect.