One way to fight ad farm extortionists would be to have land sales expire after a certain time. This time could be made shorter if the land is being sold way above market value. It could be made longer or even infinite if the land is at or below market value. The extortionists' basic strategy is to wait months or even years until an adjacent land owner gets fed up and buys them out. The exact mechanism for renewing a land sale should be set up so only a human being could do it... perhaps by putting "captchas" on a web form. (If 'bots could automatically renew the sale, that would vitiate the point of having there time limit in the first place.) This idea would also stop a less annoying scam: people who use the land sales as a means of giving their parcel a yellow color on the world map (in the hopes of bringing in land buyers who might create traffic or even spend money while on the land.)