I think they would do just as well sending tells every 2 seconds to the next person in the list.

I suggest Square target the means for RMT to exist - the buyers. Simply employ staff to act exactly as gil sellers undercover (obviously using new names periodically to not become discovered / known by buyers). This will be a trap to catch and ban many buyers and discourage many, many more to the point genuine gil sellers don't have a worthwhile market with FFXIV players.