The company behind LoL, Riot Games, has tried everything they can think of to solve this complex problem. Riot has introduced a variety of methods, from studies of the most appropriate colors for text (red for negative text, blue for positive) to collecting player behavioral data during games, and even introducing an honor ribbon system that rewarded "good" player behavior with cosmetic upgrades.
They also studied the difference in wins between players who they designated "sportsmanlike" players and other players. Turns out, the difference is 1.7 million games per day. Riot claims there is a 10% increase in wins for good players over average players, but bad players won 35% less games than the average.
Don't believe it? Watch the video below, directly from the LoL homepage: