Once you are sure that the user has had their say it is time to acknowledge their complaint by summarizing what you heard and where you believe their expectations were not met. Be understanding even if the expectations seem completely unrealistic from your point of view. Expectations come from many things, the communication from your company to the customer, previous experiences with similar products or services the user may have had, the users personal values, etc. You can manage these expectations, but only after you have acknowledged them and explicitly worded them.