Had similar idea, but not speifically against afks, but in general against "undesirables". Trolls, int-feeders, afks, e-bay accounts, toxic players etc.. In my idea you could only ban players from matchmade WITH YOU. So if you ban a player, then he might appear in enemy team still.
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problem: As we know everyone hates good players. People DESPISE when someone has skill, and take it as personal offense to be carried. Even worse when the good player is in enemy team. /s
Now to be serious. One argument is that people would ban good players, and that would prevent them from playing. Might be true on super high ranks.
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problem: Jerks couldn't play at some point, because they would be banned by everyone. And there are plenty of jerks around, that might buy skins. Preventing them from playing might mean profit loss.