I would work through your credit card services first and explain the situation since they in theory work for you so that the reversal can be reversed. (lets don't get the double negative conversation started!)
Chances are the folks may not have realized what charge they were disputing to their card company. Have you talked to your clients?
If they bought a different house than what you inspected, I don't think you can place a lien; but if the same house a mechanics lien can be put in place. Send them a letter with the threat if they are being azzholes. A clouded title can keep them from selling or refinancing.
Good Luck!

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Jim Luttrall
Mr. Inspector.net, Inc.
Allen, Texas 75002