I work in customer service answering telephones all day long. Two of the places where I've worked did not tolerate abuse of the staff at all. Give your reps instructions to give a clear firm no. If the customer wants to push the issue have the supervisor give that same clear firm no. Where abuse was not tolerated we rarely dealt with abusive customers. I did however work for a large wireless provider and we had to just take it and boy did we ever get crapped on all day long. Few customers will persist in abusive behavior if they are politely advised that it won't be tolerated.