Automatic Demotion
I've seen this way too often in rank play where Blue Team is doing well and then someone makes one mistake and one player ends up going afk and game just spirals into one hell of a game loss, or in Red Team where there is a complete toxic player and they just get reported, maybe get a warning for their "first offense".
I honestly think that besides getting banned in the future (where they'll just create a new account anyway) that there should be an on-going repercussion for inappropriate behavior --- maybe an auto demotion of some sort. For their first offense they get demoted one rank (ie: plat 1 to plat 2, plat 2 to 3, etc), and as re-occurring offense would be an actual league rank demotion (ie: from plat to gold, gold to silver, etc).
Maybe have a time-out punishment on que time.. half hr, or an hour.. maybe longer.