If i had a customer come off at me that way, i would just cut ties and let them go to someone else after explaining why in a professional matter apology or not. Sometimes trying to make a broken customer relationship work, just isn't worth it and who knows when they will decide to change their attitude again and flip out which can turn off potential future customers. I'm not trying to be against the op or for Shaun, just what i would do in this situation. Not everyone will be happy or see eye to eye in life, just how it is sometimes.