Griefing in an ARAM of all things should be an instant suspension -> ban
It's very telling when your teammate is so tilted he can't even stand 8 minutes in being behind before wanting to throw in the towel. Saying "onto the next one" or anything similar should also be grounds for immediate suspension because it shows upfront you have no intention of winning the game.
Play the stupid 18-minute ARAM out. If you can't last that long, I fail to see why we should allow that person to continue playing a MOBA. It ain't COD-paced.