I think you need to make friendly fire more severe. Perhaps 2x as much damage to the offender as opposed to equal?
I had an incident in a recent Coop match, where I shot a team member by accident, he didn't die.
The then killed me purposefully, then with some sort of childish vengeance, proceded to hunt me down for the rest of the game.
Im not going to quit the game, but i spend 2/3 of the game running away from some mentally deranged child.
As a result, I would either request a change to friendly fire, or some way of reporting morons.