Should each and every black hole decay? Is it necessary that black holes should always decay, say if we have a potential for not letting the Hawking radiation escape?
 A: Lets take the case of a Schwarzchild black hole embedded in an asymptotically AdS spacetime. We know that a geodesic in an asymptotically AdS space takes a finite amount of proper time to start from a point, reach the boundary and come back. 
Now the black hole has a finite lifetime of evaporation due to quantum effects which is proportional to the $M^3$, where $M$ is the mass of the black hole. In case the time for a geodesic to start from the horizon, go to the boundary and come back to the black hole is less than this evaporation lifetime, the Hawking radiation starts getting into equilibrium with the black hole. This is a phase transition with $M$ playing the role of an order parameter, and is commonly known as Hawking-Page transition.
You asked in the question if a potential doesn't let the Hawking radiation escape. My example meets this criteria because AdS effectively acts like a box potential, and doesn't let the radiation escape, rather the box forces the radiation to be in thermal equilibrium with the black hole itself.
