Standing one arm neck grab

[Half Boston Crab]

If it's grappling (not self defense), and you don't want to hurt the guy, tighten your neck and (if he's grabbing with his right arm) grab his wrist with both hands as close to his wrist as possible, pull his hand to the left and then hard downwards in one motion. If that doesn't work and you're desperate, you can always pull guard and push him off with your feet, or go for an armbar.