Beat a dead horse Informal

To keep spending time, energy, or words on an argument, topic, or effort that has already been settled, resolved, or proven impossible to change.

It is used to tell someone that continuing to push the same point is pointless, since no further discussion or effort will change the outcome.

The phrase draws on the vivid, almost absurd image of whipping a horse that is already dead in an attempt to make it move, an effort that obviously cannot succeed no matter how much force is applied. It gained wide currency in nineteenth-century American political rhetoric, where speakers used it to dismiss continued arguing over an issue that had already been decided and was not going to be reopened.

We already voted on the budget last month, so bringing it up again is just beating a dead horse.

The coach kept criticizing the same mistake long after the game was over, beating a dead horse in the locker room.

I know you disagree with the decision, but the board already voted, so let us not beat a dead horse.

A: Should I email him one more time about the deadline?

B: No, he already said no twice, you are just beating a dead horse.