Looking at your join date, you weren't here at launch, where there was no such message restriction - which hardly even affects legitimate players as it is now - and RMT /shout spam was in full force, to the point where it was almost impossible to hold some sort of actual conversation in the main cities. Once /shout chat began to be moderated more closely, RMT then moved into sending /tells, which in my opinion was a huge "improvement" (for want of a better word). Even if you do impose more limitations in an effort to combat RMT, they're going to find a way around it (like you yourself have said), and there are very, very few options that will not catch actual players in the crossfire.
Also, investing a ton of resources into combating spam is more or less a waste, when they can be directed towards shutting down the actual RMT operation. If we keep banning the spammers without focusing on the RMT characters that are holding their gil, or the buyers, it's going to change nothing - they'll just find new accounts to hack and spam from.
As an aside, at this point RMT spambots get banned fast enough that you generally don't have to blacklist them. And because some people still don't know, banned characters are NOT deleted - any spambot older than probably a day or two can be safely removed from your blacklist. Personally I've turned off the sound effect that accompanies receiving a /tell so I can largely ignore these messages. Worst comes to the worst, you can either filter /tell messages out of your General tab entirely, or set yourself on /busy to block all /tells, regardless of who they're from - though that comes with the downside of not being able to receive /tells from friends.
Though I only have one person on my blacklist, that being an actual player who miraculously managed to irritate me enough, I agree that some improvements to the blacklist can be made, such as deleting chunks of people at a time.