Genuinely listen to their concerns. Most difficult clients have a frustration that they merely want heard and are satisfied once someone listens to them vent. For clients that complain regardless of proposed solutions, the professional should continue to do what they can to mollify the client.
Stop smiling. No matter how frustrating a client is, it is crucial to the reputation of the spa for the skin care practitioner to maintain a professional attitude. The best way to do that is to always greet the client was a smile and continue smiling until they leave.