And how would you do that? Even games like Black Desert where there is no player-to-player trading and the system controls the market price of all items they still have regular RMT spam. Of course they also give you the ability to make your own chat filters, but that doesn't stop the RMT and bots from existing.
There's no surefire way to get rid of RMT that doesn't also involve completely screwing over your players, and even then the RMT are sure to find a way around whatever measures you put in place. Unfortunately as long as you have lazy players who have more money than sense you're always going to have RMT in some capacity.