I'll add another idea, the brutal idea. This one will receive critics because the method, but... It will work, HAS to work. As someone said, the only way for stop RMT would be try to convince everyone for not use their services... Yes, this would kill RMT from the root, indeed, but... It is utopic, if we could convince people so easy, many of the problems of this world would be solved already...The bigger a group to convince is, the closest it is to impossible.
That said, what does it works nowadays with the people? What does really stop persons, or 'convinces' them without do it? Threatening, yes, that's it, it is how our whole law systems works after all, many people don't commet crimes because they know something bad would happen if they do... Else wouldn't they do whatever they want?... Sure they would. People in this forum that is pissed off express their annoyance to Square Enix, but they don't dare to insult, is it because they are polite? Of course not, because they fear a ban. And I'm totally sure that if insults against Square Enix wouldn't be a ban, we would see many here, am I right? Actually trying to stop the drugs in some countries, they are focusing more in punishing those who buy the drugs, that those who are selling them, this would be the same.
SE could also give a big scale action against RMT just by punishing the players that acquire the services of the gil sellers. And this can be really easy to see and know... Like most of the MMORPG games, I want to suppose that FFXIV has a nice database with fine logs where you can search the transactions between players, check the biggest amounts and weirdest ones and investigate, where did that player obtain the money or how? Was it endlessly grinding in a dungeon with other four of strange names?....
Whatever I want to mean is that.. Actually SE could advise the players that hire those type of services that it is against the terms of service and their accounts will be banned. That way, taking actions against the players that buy RMT, you really make most of them drop the idea to even try it... At least, me, thinking like a player, wouldn't risk my account just for having more gil and probably a big percent wouldn't...
Thins is just... Advise players that you can take actions against them for this, and, also, prove that you have the needed tools for catching doing it... Without customers, RMT would stop with the time, and if you even fuse it with others idea for making RMT a pain because needing to manually do prompts...
Basically, it is a change of focus, instead of having someone checking if someone is RMT and then ban, have someone checking the logs and suspicious transactions. Or just checking who is earning money in an area, farming there constantly, and then investigate the transactions that one make... There are a lot of options once you have everything stored in your sql.
And remember, there are way more bots than players buying gil. So it will always be easier to discover players who buy, than discover all the damn bots and try to ban them all just to achieve nothing.