There is one very simple way to 'fix' this and it involves doing nothing.

First of all, people got to stop buying overpriced items on the MB. I've seen something sell for 50m on the MB before. The more people buy overpriced items, this encourages the seller to continue selling at extravagant prices, why? Because there are enough idiots out there to fill their greed.
If people stopped buying things at crazy prices, sellers will have no choice but to reduce the price until it's down to a price people are willing to pay. Every item on the MB has a undisclosed 'sweetspot', a pricetag where you won't hesitate to buy. This sweetspot is affected by the general gil consumption of the community. You will need to bring the market down to that sweetspot, so that people won't be left without option to buy gil through RMT.

But we all know this cannot happen. There is a reason why greed is a deadly sin. Because some may say the DEVS are too selfish to accept their own mistakes, but in reality they cannot accept their own selfish greed.