Though it may not seem like a lot to us because we still get spammed constantly, they do remove about 4k-5k or so RMT two to three times a month.
They release reports on the Lodestone each time with details of the ban count:
http://na.finalfantasyxiv.com/lodestone/news/category/1 under the heading: Actions Taken Against In-Game RMT & Other Illicit Activities (Date here).
There was actually four reports posted in Feb with a rough total of around 17409 RMT permanently banned!
Considering how much tells we still get after that amount of clean up, it hurts my brain to comprehend just how many there are in general.
I keep thinking there could be something that SE can put in the tell process, some kind of step or action that discourages automation of bots, but I don't have programming knowledge to fathom what such a thing could be.