When an Edo Tensei zombie's soul gets removed, its immortalized body crumbles away, did the Sword of Totsuka's general sealing of Nagato not make that crystal clear? What good is a brain without a body to control? Heck, you can probably even thrust the soul inside of an object as a makeshift sealing technique, like a tabi boot so that the zombie will never resurface lest properly tampered with.
We haven't even seen the Spirit Transformation Technique kill anybody; the description is all-too similar to the Yamanaka clan's jutsu, in which where shown, one with powerful willpower can defy the intrusion, as occurred in Sakura's battle with Ino. It would seem with this etchnique in particular though, the user's spirit cannot destroy another's (spirit), rather only attempt to seize control of their body and kill them, before returning to their own body. If they were to remain in another body for too long though, I dare say that soul would ultimately be forced out by the primitive host, and with no true body remaining, the Spirit Transformation Technique's user's spirit would ultimately perish.