Does buying groceries next month really depend on collecting a fee from this one client?  When someone starts telling me how to do my job, I advise them to find someone else to educate.  This situation sounds like it falls into the "everyone else is doing 85 on the Interstate, and the Highway Patrol can only stop a few of them" category.  If I have to deal with difficult clients, I prefer them at least to be those who drive 10 miles below the speed limit.