This is a pay to play MMO. The gil sellers have costs for every account they set up (regardless of whether they're legitimately bought accounts or stolen, there are still costs for doing either one). SE only has to slow down the gil spam far enough that the gil sellers are unable to recoup that cost and make a profit. Once the gil sellers stop making a profit from these spam accounts, then the spam will stop.

SE has the means to slow down the spam far enough that it would end up stopping altogether. They know perfectly well how to do it. Yet for some some inexplicable reason have deliberately chosen not to, at least not for their NA/EU customers. Much as I appreciate SE's skill at producing fun and engaging games, this blatant disregard for their customers is rather appalling.

And yes, no matter how fun the game is otherwise, being constantly harassed the whole time you're playing it can ruin the whole experience. Nobody will ever know exactly how many customers SE has lost over this issue, but I'm sure it's a lot, and they'll continue to lose more the longer they allow the problem to persist.