I think there may need to be a distinction between pinning and stacking.
For me pinning is stopping the robot from moving but it is still on it's wheels, and stacking is when the robot is against the barrier or something similar, and the robot is on it's side or back or has been lifted so the wheels are off the ground.
I suppose the problem arises when a robot is stacked but will only remain stacked by pinning.
I think this constitutes pinning.
A simple answer is after 10 seconds the pinning robot must reverse about 3-4 meters back or to the centre of the arena which ever is the shorter and then can re-engage.

Bad sportsmanship is ambiguous, I know what it means to me, but I don't think you can get a universal definition that covers every single situation, so I would tend to use it in the code of conduct guidelines and not in the rules section.

but that's my opinion.