Ban by HWID. Client-Server Software
Client starts software that generates unique HWID of particulat Computer and it hooks to wow.exe
Server software checks correct checksums of client software ( safety precautions because of reverse engineering ) and checks if there is HWID - lets client connect to server.
When gold seller/hacker/cheater gets ban hammer he need to change hardware component to get the new HWID because reinstalling system or changing IP/MAC/VPN doesn't work. And by coding choose most expensive hardware component for generating HWID, because vanilla wow doesn't need super-ultra computer try to do HWID generation more sophisticated so they cant change one component to get new HWID. Next step implementing PIN Generator to protect accounts, vanilla wow has this built in but they never used it.
If this isn't possible for WoW then there is no salvation.
PS. My account is already banned for unknown reasons to me, i didn't even play the game after transferring account :)