I checked the FAQ already and couldn't find what I was looking for.
In Magic in the Shadows, in the "Spirits" chapter, it says: "If the victim is killed or knocked unconscious, the spirit is unharmed, but it is driven back into astral form."
Meanwhile on page 13 of Critters, it simply says that "if the victim is incapacitated (suffers Deadly Physical damage) by Physical damage, the spirit is unharmed but driven back into astral form."
So does Deadly Stun damage drive a possessing spirit out of its victim? My gut tells me that MitS takes precedence over Critters, but I was wondering what you all thought.