Maybe it'll help here to point out that if you start with a spinning black hole there's an easy way to stop it spinning. You just chuck in matter with the angul

Maybe it'll help here to point out that if you start with a spinning black hole there's an easy way to stop it spinning. You just chuck in matter with the angular momentum going the opposite way to its spin until all the angular momentum has cancelled out. Then you can do the fancy stuff from this paper with your new spinless black hole.