Fixing multi client wouldn't stop them from wintrading, because they can do it with their friends still.
They're also creating dozens of characters with free vouchers etc. and benefit from it even more, that's why they should just remove hongmoon coins from season rewards in pvp ladders as far as I know other regions don't even have HM coins in their pvp ladders, but we still do for some reason. That would be at least easiest fix now, then there wouldn't be any reason for wintraders to wintrade if they don't benefit from it and to keep PvP alive they should just updated the PVP shops to give, useful stuff for example weekly pet/pods vials.