DITTO Adam.
When I was doing driveways this heppening all the time, along with the Steady calls for "Do you know it is snowing and there is 3" of snow in my driveway"?? It is not different with lawn service, and as you add customers the averages increase, 1in10, 3in20, 4in30, and so no. That is why I am no longer doing any type of residential service.
I agree that standing behind your work is key, but when you have the same customer moaning all the time it is becomes not profitable to keep going back even when they are paying, and it dosn't look good for your business when they keep moaning to all their neighbors too.
Sound like you already know what to do if the calls keep coming in. The only thing you have to ask yourself was this a refferal customer or a cold call. Most of the time refferal customers are not moaners. Good luck.