You want to get rid of Gil selling spam?

Then get SE to produce an in-game item that gives you 30 days of paid game time. Make this item tradable (and thus sell-able), but also make it only initially available through the Mogstation. IE - you want Gil, buy one of these and sell it on the board. Someone prefers to spend Gil instead of cash on their subscription? That person buys your subscription item and uses it.

Once that is done, market forces (supply and demand for this item) will force the Gil sellers out of FF14 somewhere else. It won't happen overnight, but it will happen.