Firstly, the fastest way to make an annoying gil seller leave you alone is to blacklist them. This can be difficult when they are spamming super fast as it's hard to right click their name. On this front, I would suggest a small forced delay between posts in public channels. Not a huge wait, but a small one, maybe a 3-5 seconds.
Additionally, I'm sure that this has been suggested previously, but a right click + add to blacklist option would be great!
For the anti RMT task force have some way that you guys can collect data on who is blacklisted. Maybe use a tally system. This system could flag accounts who have been put on a large amount of blacklists. Once a character has been put on 100+ blists the account is flagged to be investigated. I wouldn't suggest an instant ban because it's always possible it's just a troll... but then who really wants to keep those either?
Please Like or comment and suggest your own ideas to help out with the RMT problem!