Square Enix is simply lazy with their programming. Dozens of other games have come up with simple solutions mentioned by hundreds of previous posters on these forums for the last year plus. Simplest of them all is adding a "report gold spam" option to right clicking someone's name in chat who just sent you a RMT spam. Another very simple line of code or two to notify a GM that a single account has gotten a dozen or more of these report gold spam flags in a short period of time and shazzam, ban gold spammer.

Biggest problem, in my personal opinion, is that SE just doesn't care enough to fix the problem. Other games do it better, why can't SE?