This is already the punishment for being found guilty of buying RMT services.
So, tell me, how will you perfectly detect all of these buyers? How will your customer support team be able to receive and respond to alleged reports of gil-buying while still being able to filter out trolls? How will players who are simply very good at hiding their activities be caught?
Logically, the returns on finding these extremely-hard to detect buyers vastly outweigh the costs. You would have to be able to detect a significant part of the demand market flawlessly and kill it flawlessly. The fact that banning buyers is impractical is why the modern MMO market, across 15 years of technological advances, hasn't killed RMT. WoW, a project with millions in profit hasn't done it yet, you expect FFXIV to?
You want to kill a buyer market? Let's talk econ.
Reducing the visibility of RMT tells makes it more difficult for the buyer market to find resources to buy. The risks of stumbling into illegitimate or irreputable sellers greatly increase when you use Google instead of in-game advertising. Most players will then think twice about giving CC information to a seller they don't know the reputation of.
Making it more difficult to send RMT tells and shouts increases the cost of operating an RMT operation in FFXIV. If you can't send /tells or /shouts till you're level 10 or 15, suddenly the process of creating spambots takes magnitudes longer. This means you're paying the same amount of magnitudes less advertising return. Guess what this does?
It raises the price of gil. What does raising the price of gil do for a buyer market? Everything! Higher costs mean less demand. Less demand means the RMT companies overall get less return, thereby denying them resources to re-invest into advertising. Eventually they'll stabilize - but at much less visibility, with a much smaller buyer market, and less profit! Which means...
At least a portion of their effort moves to other games.