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What I am about to propose may seem harsh but with the fact that it seems you get more leavers/AFK in ranked then in normal (at least I have, and I don't understand why) you should eventually remove their ability to play ranked games at least for a while.
You can have the normal suspension and then an elongated term of no ranked until you have redeemed yourself in normal.
I also think this should be more of a leaver thing, but it could work for trolls as well.
p.s. I don't have very many ranked matches on this account specifically.