If your support trolled even once, enemy ADC gets 2-0, while you get 0-1, you can close client.
Game is done.
There is no meaning in playing after that.
So I think any punishments from leaving games should be removed, and should be removed any punishments for leaver.
For example, ADC played honestly but his support/jungler made enemy ADC fed, his lane is lost forever.
No point in playing already lost game.
You can count scores up until the point of leaving and not punish players who already lost a game even more?