Thats unusual behavior for a Kole, they are concidered the less aggressive of the tangs. Who does he pick on the most? Is everyone getting enouph food so they don't feel like they have top compeate for every morsel?
Maybe just feeling a little crowded? Have they all been together long? Not much you can do but remove him or the fish he don't like. I finnally had to pull a mean yellow tang after giving him months to calm down. Hated to see him go but had no choice for the happiness of the tank.