The only leverage you might have now since they are deneying it and already have their money is to start placing bad reviews of them online.
Top review sites are Google, BBB, Yelp, Houzz and Facebook ... read more
Another option, which I have not used myself, is used a lot in Houston. https://www.plylox.com/ This company sells a clip that is attached to 1/2"plywood, and you can typically get them at your loca ... read more