An easy way to prevent ranked AFKs
A player's first ranked AFK should trigger a one game ranked ban and a nastygram warning them not to do it again.
Every ranked AFK from that point on, should double the duration of the ban and send another nastygram.
2nd AFK - 2 game ban 3rd AFK - 4 game ban 4th AFK - 8 game ban 5th AFK - 16 game ban
And so on, with no upper bound.
By the time a player AFKs for the 10th time, they'd have to play 1023 penalty games without going AFK to get back into ranked.
Because of the slow build on this system, it would only really hose habitual leavers. People who have intermittent connection problems might get a 1 or 2 game ban, but overall would not really be harmed.