To quickly establish communication and resolve a problematic situation with a customer, you must first actively listen and show understanding. First, admit your mistakes and ask for forgiveness without being provoked. Then, try to end your conversation with the customer on a positive note.
However, if your customer constantly breaks your company's rules and misbehaves, you can block him. For example, in EasyWeek appointment software, you can stop specific customers so they can no longer make an appointment in advance.