The same way people who think that gold selling (or really any interaction where a player exchanges real life capital for a service) is solvable: Hire an infinite amount of GMs to work 24/7 to tirelessly eliminate the very idea of gold selling from the vernacular. Nevermind the fact that this is 100% unrealistic and has many, many downsides, to these people Blizzard is a multi dollar company so all they have to do is throw money at a problem and it will disappear.