Not much you can do against cheaters in pve. Only thing you can rest on is that these people don't know how to play games / can't be arsed to play them, same with pvp cheaters and are pathetic enough to rely on automatic commands or prompts to "play" the game.

One way to hit RMT hard is to have a clear "ONE FC HOUSE PER ACCOUNT" and have all your alts share one house rule and nuke the people's ownership who own full wards of submarine churning FC's.
I own a single FC house and it churns out 500k-1.7 mill per every two days. Do the math on how this might multiply if you own a full ward of houses.

Yes, this includes nuking people who had houses BEFORE the changes, no loopholes, no "Its unfair, because I got the houses before the changes " no mofo, you probably have goldcap multiple times over, there is not a single thing in this game you can do in this game with that gil to make it go away. EXCEPT SELL IT FOR REAL MONEY, which you probably are doing.