Rayphoton

Our small group has this argument and we cant find a super clear answer. Crossbones (or kingpin or hullbuster)has "inured to pain" (or equivalent)and is sitting on 5 power. He gets hit by a single 5 damage attack. Can he spend 5 power to reduce that to 0, or..can he only spend one power to reduce it by one and has to eat the other 4? This will help clear up a reoccurring argument our group keeps having. Thanks Ray