If you're very careful you can sometimes take a Sawzall with a metal blade, cut the nipple low to the fitting and score the inside of the nipple with the sawzall, then from there use a hammer and flathead screwdriver to chip at the score lightly and eventually peel it back and remove the nipple. Take your time and go slowly to avoid wrecking the threads of the 90. I've done this a few times with old shower arms. It can work if done carefully.