Which "problem" would you adjust first as developer?
There are many unsportmanlike, annoying, or even gamebreaking stuff the players do. If you were given the chance to provide harsh punishment for one of these acts, then which would you chose?
"easy": Writing easy at the end of the game as winner.
leavers/afkers: obvious. Camping on start also considered as afker.
surrender spam on ranked: early lose makes some people spamming surrender, and often followed by flaming when declined.
low pts champion on ranked: Bringing a champion on ranked you have low points with, and can't use it due to lack of experience. No problem on normals!
Ignoring nexus when it's vulnerable.
"non-int" feeders: People who cannot understand, that they doing it wrong and die to the same stuff over, and over again. Such as the push+die, or dive+die, or go out from turret against obviously stronger enemy.