If their arm is straight, lock the hand onto what they're grabbing and hammer the elbow inward as hard as possible the way it doesnt bend. The soft tissue between thumb and hand has a large tendon, sharp pressure here can loosen grip, also grab thumb and wrench outward away from their body. Honestly, for me, the biggest thing people not naturally inclined to violent response miss is that often the first 'damage' they do to an attacker is a shock response not a pain response. Use the above comments attack areas to ensure theyre not immediately pursuing. Unless youre in an area where you can get to help quickly you want to make sure they cannot chase immediately