I cuss. A lot. But never have I dared to cuss at a person in a customer service role. And I get rather upset about lousy customer service and still manage to talk nicely to people about any issues I encounter. Some people cuss, and yell and make threats when they are angry about customer service. This is not okay.
You have to draw the line on unacceptable behavior with customers, just as I hope you do in your interpersonal relationships when people disrespect you. You get the behavior you tolerate. So, don’t tolerate profane language.
Diplomatic comebacks to cussing set you up as professional and assertive, and they help you get the respect you deserve.