The best way to stop RMT is by stop buying Gil from RMT.
The best way to stop RMT spam, is creating a code that prevents users from sending /t ever minute or so. Unless on friends list.
Shouts and /say would have even greater restrictions. (I rarely use them anyway)
Market place is fine, just have someone(s) monitoring sales. No one is going to buy a swan feather for 100,000,000 So unless the RMT goes out and makes HQ 5 materia gear and sells it (which is total legit)
Of course if someone buys it for more than standard market price. Red flag.
You will never get rid of RMT, but you can stifle it. Keep it contained. Right now, SE has bigger things to worry about. RMT on FFXIV isn't nearly as bad as some other games. there is nothing to buy anyway so spending USD on gil is like levelling multiple characters for story rewards. waste of time.