I didn't see anything specific about devouring souls in the bestiary monster description.
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Make it a gold prison: make him the warlord dictator of a minor country and fund his armies in training, equipment and recruitment propaganda tools.
Pick fights with him a few years after when everyone has forgotten he was put there by you.
Create a secret spy agency to train his spies, then have your spies meet his spies when you want to tell him where and when the next fight will be, and who the fall guy will be.
If his old friends reach out to him, well then that won't do, and you make sure he knows that his friends are the next fall guys.
Instead of "killing" him, "convert" him. Pick your flavor of change, but if he becomes anti-old_self, then he no longer a problem.
/cevah