Unfortunately since a spinning black hole interacts with space then as it spins it will lose energy. As it loses energy it will slow down. It will take a long time, but it will slow down. The interaction I am thinking of, which assumes the black hole is not interacting with gas and dust, is based on frame dragging. Frame dragging has been demonstrated. So as the black hole causes space around it to twist it will dissipate energy. At least that's what it looks like to me. I'm not sure if a spinning black hole generates gravity waves, but there is a good chance that it will. Gravity waves of course would definitely take energy from the rotation of the black hole.

I have no quantitative calculations of this, it is an extrapolation of the fact that nothing spins forever. It always interacts with something that will cause it to slow down. Of course what happens is that the angular momentum of the black hole will be shared with everything else in its vicinity, since angular momentum is conserved.

Edit: I forgot one effect that would definitely take energy from the black hole. When light passes a gravity well it is deflected. That requires energy, which has to come from the black hole.

Bill Gill

*Edited by Bill (08/26/15 11:19 PM)*
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C is not the speed of light in a vacuum.

C is the universal speed limit.