First off, don`t forget that rotating black holes are thought to have ring-shaped singularities rather than points. In fact, it might be simply because of the light-speed rotational velocity problem that this is believed to be true. Anyway, here`s two quick ideas on why the rotational velocity won`t increase as much as you might think:

1) As the rotational velocity nears the speed of light, the equations needed to figure the acceleration are going to be affected by relativistic effects - it will require much more initial momentum than classical physics would predict to result in a rotational velocity near light speed.

2) A rapidly spinning neutron star loses momentum through its radiation acting as a kind of magnetic brake (sort of like a car alternator putting a load on the engine) and possibly through ejection of mass.