Some clients cost more than they bring: repeated no-shows, an argument about the price every visit, demands that are not achievable, abuse of you or anybody else. Keeping them is a decision to be paid less for a worse day, and you are allowed to stop.
Try the smaller fix first where it is warranted. A deposit on every booking solves the serial no-show. A written quote before each appointment solves the client who disputes the price. A consultation appointment solves the one asking for something impossible. Many difficult clients are a process problem wearing a personality.
When it is genuinely over, say it once and keep it short: you are not able to take their bookings any more, you wish them well, and you can suggest somebody else if that helps. No list of grievances. A long explanation is an invitation to argue each point, and there is nothing to be won.
Then act on it — stop offering appointments and decline further requests rather than leaving it ambiguous. Honour anything already booked and paid for if you can, or return what they paid; ending it cleanly matters more than the last hour of income.
Abuse, threats or harassment are a different matter and need no notice period. End it immediately, keep the messages, and report it to us. We can act on somebody's place on this platform, and we would rather know.