I don't know if anyone else has posted this idea so if you have I'm sorry, I'm not trying to steal your idea...

When you identify a gil seller, do some behind the scenes stuff in your systems so that they log in to a copy world from that point forward. A prison if you will, but the catch is, don't tell them, make the change without notifying them. Stick all gil sellers in these prison instances together so the only people they can shout or tell to is each other. You should be able to do this because they broke the rules in the first place so you COULD ban them, and since they still log in you still make money off of them, but we don't have to interact with them anymore. Win win. Eventually they would realize of course what has happened and they would delete their account or complain or whatever, but at that point who cares, they broke the rules anyway and none of the normal populous even interacts with them anymore so we wouldn't even notice... which is the point :-D