***** NOT A TEACHER *****
Mona: I think that Mr. Smith would be a good manager.
Raul: No, he wouldn't.
Mona: Why?
Raul: He is very rude.
Mona: He is only rude to people who deserve rude treatment.
Raul: No one deserves rude treatment.
Mona: I think that some people do deserve rude treatment.
Raul: What kind of people?
Mona: That would take a long time to explain.
Raul: Oh, Mona, I'm sick and tired of beating a dead horse. = I am tired of discussing this with you. You believe something. I believe something. You are never going to change your mind. I am never going to change my mind. So let's not talk about this topic anymore.
P.S. I believe that there is also a somewhat similar saying of "When the horse is dead, get off." (For example, you think that young people should read print newspapers. But most young people do not want to read print newspapers. They read the news on digital devices. So stop telling them to read print newspapers. You are wasting your time. Accept reality: the horse is dead. Get off.)