Basically, PC bangs don't require players to own a copy of the game to play, so they can create free accounts, and when they're banned on one, they can just make a new one and continue to cheat on the game, but as of February 17 this will change.
Blizzard announced that a Korean Battle.net account will be needed to log into Overwatch in the future, and these require a social security number. They hope that this will be enough to discourage players to cheat, as this will make it a lot harder to set up new accounts once banned. Players using foreign Battle.net accounts will have to purchase a license for Overwatch, however, which should also stop those in Korea using NA or EU accounts to get around this.
At the moment only Overwatch is facing this change for PC bangs, but Diablo 3 and StarCraft II will soon get this too. Will this be an effective deterrent, or does more need to be done?