That's being a total asshole but if you wanna derank at least play normal and then probably just mess up in a teamfight and cost your team the game. At least better than being a complete troll as your teammates won't be as frustrated.
easiest way to derank ive found was to play jungle, get doubles and feed them to bottom lane. one big death like that completely throws the lane and you can continue for mid/top. wont set triggers off in riots ban system cause its just a few deaths also.