LoL needs a mercy rule

just played a game where we lost 15-50. one guy openly admitted he was punishing us by not surrendering. the other guy just said 'I do not surrender'. the other team had a 19-2 ashe who was trying not to end and telling his team in all chat not to push or take turrets.

If I leave this game I am hit with lower priority queue so I am trapped in hell for 15 mins where any time I venture outside of the inner turrets I am jumped on by 5 people and laughed at and taunted.

there should be a mercy rule. if the other team is up by 20 kills just let that be victory.