I'm going to jump in quick to explain how to make this un-abusable for the sake of future comments on this thread.
Since the likely solution to avoid this is duoing or smurfing and sending one person from your group afk just in case, I propose this fix:
The person who left the game gets increased LP loss and the duo partner of that person does not get the reduced loss. This means if you duo with somebody to prevent losing large chunks of LP, it is not going to help you and you now have to really trust your duo partner not to leave, as if you choose a duo partner who decides to leave mid game, it's partially your fault for playing with him.
Secondly (I don't know why people think this works but I'll bring it up), people trolling or flaming to try to get somebody else to afk. There is a few parts to this. Firstly, the troll should get reported and eventually punished (but as we know that doesn't just magically happen there is another solution). The person who afks gets a greater LP loss, as stated before, so if you feel so down because somebody is yelling at you or because you know you arn't going to win, YOU are sacrificing your own LP for it, and ragequitting is just as bad as trolling imo.
Sorry if this is a bit long, I try to keep my posts shorter
