You should have told him the real reason why you weren't doing them again, that's unless you don't still do the area, don't let them call the shots.
I cleaned today and the customer said he'd had some new d/g panels fitted, I said the insides need cleaning as they were all smeared, he said the fitter had done it with some glass cleaner, they were really bad, someone had used windolene which is difficult to remove with water based product.