*sigh*
The solution for this is for fewer people (I am being polite) to be part of the problem. If you're on a server with tons of RMT tells, it's because there are sufficient numbers of idiots (oops, not so polite now) willing to buy gil on your server to make it worth their while advertizing and producing gil.
Frankly, I think the problem is that these RMTs are like cockroaches, and a new one will spring to life to replace any that are removed. Rather than attempting to kill off the roaches (which is norotiously difficult), we should be targetting that which these roaches feed on, in other words the players stupid and annoying enough to buy gil.
I believe that every time they ban a RMT account that has been used to pass gil on to gil buyers, they need to trace back through their transaction logs and identify every account that received a non-trivial amount of gil and do something with those accounts. Personally I favor confiscation of all gil and busting their classes (all of them) back to lvl 10 at the most), I don't think a temporary ban is sufficient deterrent to gil buyers. Also there is no fear of the consequences from buying gil. If you knew one of your friends bought gil, would you be happy about it? Anyone in my FC who ever buys gil will be summarily dismissed with no opportunity to rejoin - ever.
The problem is that buying gil is not seen as socially unacceptable. I see the problem almost like speeding. The thing about speeding is that it's kind of a socially acceptable offense. So much so that if you obey the speed limits, you are generally one of the slowest things on the road, and people deride you for being slow. Until speeding becomes socially unacceptable, it will continue to happen. Once it becomes socially unacceptable, speeding will diminish.
Buying gil needs to be seen as unacceptable by the vast majority of players, and those who do it need to be shunned and punished. A combination of deterrence and social rejection would quickly limit the number of players willing to risk the consequences of buying gil. It's just that no one wishes to hold gil buyer's responsible for their actions. So they continue to give RMTs the incentive to bug the living crap out of the player base. Whether they like it or not, the people buying gil are DIRECTLY to blame for the continued presence of the gil selling cockroaches called RMT. I see gil buyers as no better than the RMT, they're cockroaches, just like the parasites that sold them the gil they bought.
</rant>
p.s. I'm on Behemoth which has a pretty heavy contingent of RMT spammers right now, it is a royal pain in the ass.
P.P.S. I agree with those above suggesting a friends only option for "/tell" or a whitelist of people who you will accept tells from. It's a good idea but has a downside in that it restricts players from communicating with people they are not in the same location as, unless they are already in the same LS or FC.