I say that this gil sellers spam is annoying and i know one thing....if nobody buy gil's then there will be no gil sellers, easy as that.
I play Archeage before and there was something like recognizing who got and how many gold from gold sellers site and they simply take away that gold so those who spend money for cheating spend them for nothing (Trion even suggest to go to that sites and ask for repay ). I simply suggest something like this for FFXIV, strike at the gil-buyers cause they have same guilt as the gil-sellers.

If nobody buy gil then gil-spam will be no more or very limited cause this will be unprofitable for gil sellers.