Don’t be so touchy. Complaints qualify as feedback, just more spirited. More valuable as well, because complaints are like giant signposts pointing to the areas of your business where there is room for improvement. It surprises me how often designers will attempt to strike back when anyone has the nerve to complain. Even when the client is wrong, a complaint indicates that we need to communicate/educate better to avoid misunderstandings.
By the way, we all have clients who complain all the time. Those may be ignored.