It is tempting to fire angry, demanding customers. Customers don't understand the complexities of billing systems, vendors, processes, or team structures. At least, they don't understand *your* challenges with those things, only their own.
The average angry customer understandably wants you to deliver on the promises the business has made or implied. The most demanding of these customers are pushing for the best version of those promises. Viewed in this light, learning from demanding customers can improve your services. But, very rarely, a customer must be fired.
1) The customer who abuses your team. I don't mean customers who express anger or frustration with your company. A customer who curses at employees, or who threatens, harasses or harms a member of the team can't stay. They will pull the team down and prevent you from helping other customers.
2) The customer who needs the impossible. We can't violate laws of government, physics, or time. I remember an unusual situation with a customer whose expensive cable box was replaced many times. Eventually, a technician noticed that the customer's home was on a different power grid than our cable system. It meant that there would always be serious power problems and cable boxes would continue to short circuit. We had to say goodbye.
3) The unprofitable customer. These customers reduce the ability to serve the rest of the customer base. Causes can range from overusing customer service to poorly designed discounts or high startup costs. As you learn the business better over time, you may find specific customers who don't bring enough revenue to cover expenses. Raising the price will often solve the problem though sometimes the customer will then leave. If the choice is between keeping an unprofitable customer or losing them, the choice is clear.
In my experience, briefly tell the customer the reason why you will be unable to help them in the future. Set a deadline and make a genuine offer to help them move to another service. If the customer was abusive, protect the team and make assistance conditional on professional behavior from the customer. That's it and then follow through with what you said.
Treasure the malcontented customer, the one who expects more from you on a better schedule. Seek to please the silent majority and keep them coming back for your services. But let go of the customers holding you back.