[rotorwrench]

You may have luck with a properly sized cape chisel. Get one that is more narrow than the width of the woodruff key or grind one to the proper width and start working it at one end or the other to make a divot for other punches to seat on if the chisel doesn't get it to move. Flat keys on a straight or tapered shaft have to slide out but a round bottom woodruff key has to rotate out. When it moves, it should come out. If that doesn't work then a dremel or other rotary grinder type tool may have to be used to grind it thin & peel it out.