All pool players know what a runner is: you agree on playing best of 3, and then after one game your opponent leaves. I think that to solve this problem there should be some sort of best of 3 button. Perhaps when you challenge someone, it could say "This player challenges you to a best of three", or once the game starts you can send an official best of 3 request. Either way, once the request has been accepted, you cannot return to the lobby until it is completed. Just an idea.