Add an option to leave custom games, I cannot believe this hasn't been done already

Add an option to leave a custom game, I cannot even begin to explain how frustrating it is as I am right now writing this that I had a player feed 8 kills to the enemy team in a ranked game without getting a single kill or even assist of a kill in 30 minutes of a ranked game and afterwards attempt to talk to them in a lobby which they immediately start a custom game which i am now stuck in with two players because i was under the impression you could leave a custom game whenever you wished, to have two people waste 35 minutes of your life and then another who knows how long afterwards is simply dumb and should be removed from the game. I honestly cannot believe there is no option to leave a custom game