You can say "have a nice day' in a way that tells the customer to get stuffed, or in a way that indicates that you mean what you say.  It depends on the WAY you say it, and the context.  Cooperative tones of voice have one characteristic in common.  When a person speaks in a calm, cooperative way, they usually stress only one or two words in a sentence.  When we talk about stress, we are referring to verbal emphasis on words.  When a person speaks in an angry, frustrated or confrontational way, they usually stress more words in each sentence.

This is an excerpt from the Defusing Hostile Customers Workbook.  For more detailed help on appropriate tone and word stress or defusing customer hostility, click here.