In regions of space where there appears to be a vacuum, a mysterious effect called vacuum fluctuations occurs. These fluctiotions manifest themselves physically as the creation of virtual particles (that is, particles with energy but no mass). However, the law of conservation of matter and energy states that matter and energy can neither be created nor destroyed. This means that when these particles form they leave another regionwith negative energy. Normally these particles collide and destroy each other, bringing that particluar region back to 0 energy. But when one of these particles is pulled into a black hole, separating it forever from it's complementary particle, that region of negative energy that was left from the formation of the virtual particles now has only one of the particles that were created, eliminating the possibility of recovering its energy from those particles. The black hole, however, sucked one of the particles in. The other particle, which has positive energy, is now shooting off through the universe. Effectively, this means that the black hole has caused the creation of a particle, and the necessary energy must be taken away from it. So the black hole loses that much rotational energy. Now this results in a loss of mass as well, for Einstein's famous equation E=mc^2 shows that matter and energy are actually the same thing. So after innumerable repitions of the above event The black hole will eventually have lost all of its rotational energy and hence it will have lost all its mass.