  Black hole (BH) spin can play an important role in galaxy evolution by
controlling the amount of energy and momentum ejected from near the BH into the
surroundings. We focus on radiatively-inefficient and geometrically-thick
magnetically-arrested disks (MADs) that can launch strong BH-powered jets. With
an appropriately chosen adiabatic index, these systems can describe either the
low-luminosity or highly super-Eddington BH accretion regimes. Using a suite of
3D general relativistic magnetohydrodynamic (GRMHD) simulations, we find that
for any initial spin, a MAD rapidly spins down the BH to the equilibrium spin
of $0< a_{\rm eq} \lesssim 0.1$, very low compared to $a_{\rm eq} = 1$ for the
standard thin luminous (Novikov-Thorne) disks. This implies that rapidly
accreting (super-Eddington) BHs fed by MADs tend to lose most of their
rotational energy to magnetized relativistic outflows. In a MAD, a BH only
needs to accrete $20\%$ of its own mass to spin down from $a=1$ to $a=0.2$. We
construct a semi-analytic model of BH spin evolution in MADs by taking into
account the torques on the BH due to both the hydrodynamic disk and
electromagnetic jet components, and find that the low value of $a_{\rm eq} $ is
due to both the jets slowing down the BH rotation and the disk losing a large
fraction of its angular momentum to outflows. Our results have crucial
implications for how BH spins evolve in active galaxies and other systems such
as collapsars, where BH spin-down timescale can be short enough to
significantly affect the evolution of gamma-ray emitting BH-powered jets.
