Learn to mute people. Also, don't retaliate by going AFK or stooping to this player's level and breaking the same rule you're accusing them of. That really makes you no better than them.
I know what it's like to deal with people breaking rules of custom games. I'm a longtime ARAM fan, and was playing it back when it was just a custom game played on SR. Rules varied game-to-game, depending on who was hosting. But rules were always just done by an honor system. Couldn't do anything to force players to not leave mid lane, keep Ez or Lux from farming wraiths with their ults, make people sell soulstealer and the like if they bought them, etc.
But some people just don't honor the rules. When that happens, you need to bring it up to them, but don't do it in a hostile manner. If they honestly don't think they broke a rule and you think they did, just agree to disagree and move on with the game. There's nothing you can actually do about it. If they're purposely breaking the rules, discuss with the others in the game over what to do. Back in the days of SR ARAMs, fairly often a team with a player who leaves mid would be willing to just sit back and let the other team push out and win the game.
