Heat can be your friend here. About 5 years ago I started heating stubborn lens rings with great success. I have an old platen style print dryer onto which I set the shutter and let it heat up for 15 to 30 min or so. It will be too hot to touch. I use Playtex-style gloves. They have some insulation to keep from getting burned and the cheaper knock offs are made of a very 'grippy' rubber. Grab and twist.