Riot, why don't you do anything about leaving?
This is kind of a rage thread so forget anything that I say, but last game I've had a Veigar who's been playing for the majority of the game, so why would we ban him? Well, near in the end, the enemy had gotten baron and Veigar had started a surrender vote shortly thereafter. The surrender vote ended in a 3/2 as I thought we can win this game, but after respawning, Veigar didn't even move at all. No disconnect announcement whatsoever. He hadn't moved and we needed him in the fight.
Riot, why wouldn't you do anything about these types of players?