I think that discounting elo-points (something from 2-5) should do the trick, given that most people ragequit because of the rank (according to my experience). I'm quite sure that this would be easy to implement, too.
I find it unnecessary to ban a player due to ragequitting, though. Simply because it can happen unintentionally (even quite frequently if you have, say, disconnection problems). An idea would also be to subtract a certain amount of tc for each ragequit, to scare away folks who don't care about the rank.
If a player quits during a final match of 0a tournament I'd suggest that the other player wins it, if he has atleast 2 wins before that.