First, when open carrying, I firmly believe you have a responsibility to yourself and those around you to have extra 'situational awareness'. If someone 'grabs' at your gun, you are at fault for allowing them to get too close. I think you should have at least a level 2 (Serpa-esque) holster. That being said, learn some defensive moves to keep the weapon under your control. Sweeps, etc. There are moves out there designed to do just that. I took an afternoon class running different scenarios. it really opened my eyes. Get physically fit - at least enough to wrestle it away. YOUR firearm may not be completely out of your control, and you NEED to be able to get it back. If you cannot take a class, at least ask a LE/DOC friend if they can teach you what they have been taught - better than nothing. If someone 'grabs' at your firearm you can safely assume they have ill intent.
A) If you keep control of the firearm, assume the worst: draw, open distance if possible, point, "STOP, GET DOWN" (he may be unarmed now, it would not do to shoot an unarmed man). If they advance, you can assume ill intent, shoot. Have someone call the cops, and you call your local firearm lawyer IMMEDIATELY(he should be on speed dial on your phone already)
B) If you are losing control of the firearm, DO NOT LOSE ALL CONTROL. Do WHATEVER it takes to get it back! When do get it back: back up if possible. Assume it has come out of battery. Slap, rack and point at the offending party "STOP, GET DOWN" (now, he may be unarmed remember). If any furtive move is made...bang. You can assume that he is trying again to access your weapon to use deadly force. Have someone call the cops, and you call your local firearm lawyer IMMEDIATELY(he should be on speed dial on your phone already)
Open carry is no joke, and it should not be taken lightly. Train for all scenarios. KNOW YOUR FIREARMS LAWYER!