I've actually diffused such a situation before when I really wanted the win. We had one player who had gained a lead and was stomping the enemy, but then started threatening to forfeit when different teammates weren't following exactly along with his orders. One of my other teammates got really pissy and then started going off on a slew of insults of their own. The first player fed a couple of deaths and then sat at base and demanded an apology. To nobody's astonishment, this was not forthcoming.
At length, since our team didn't have enough power to force the end despite being at the inhibitors at that point, I entered the conversation with these kinds of phrases.
"Let's just end the game and move on with our lives."
"This could be done right now if we had everyone here."
"We need you to finish."
"What does it matter? In two minutes time you never have to speak to each other ever again."
Astonishingly enough, after a little bit more whinging about how he needed to teach such and such player a lesson, the afk player felt vindicated by the struggle without him and rejoined so we could quickly close out the game.
Essentially by taking a neutral stance and opening up the perspective that it would be a favor to simply play the game for me, I allowed both players to continue to blame each other but feel self-satisfied that someone actually wanted their participation in the game, I guess. Egos are a funny thing. I occasionally find the most assholish players just have this warped sense of honor and they will somewhat relent to "help" someone who has not given them a reason to continue being a target of their wrath.
Normally I wouldn't put in as much effort as I did in that one situation, but I find a couple of lines of neutral dialogue is pretty helpful in any situation. The key is understanding, like I said to the players in my match, that you will not have any future relationships with your allies after the match is done.
-Never take sides between bickering allies.
-You don't need validation of your own feelings from strangers. Especially not completely unhinged strangers.
-Strangers will never listen to a lecture from you. It's not your obligation to teach them anything, be it manners or gameplay.
-Encouraging players in ways unrelated to a debate makes everyone more willing to work together, even if they have a beef with another player.
You can always just take the easy path of ignoring cantankerous players, take probably a loss on the chin, realize it's not really that important, and more quickly move on to something better with your time.
