Decrease the elo rating of the Ragequitter by a set amount and ban them from the room for like 3-5 minutes (to prevent the instant "Haha you didn't get a win" remarks).

A deduction of 2.00 elo points per ragequit would probably send a good enough message.

As for the "But what if I'm in the middle of a match and I suddenly have to leave because it's past my bedtime/it's time for dinner/my whole family just got hit by a runaway aircraft carrier?" excuse, you place a lot of other things on a higher priority than a computer game. Good for you. These deductions shouldn't hurt you that much then if that's the case.

Time to learn time management skills.