You use the term ‘Beat a Dead Horse’ to indicate that some action or ongoing
argument is useless.
Example of use: “He already told you no; don’t beat a dead horse.”
The origin of the expression ‘beat a dead horse’ comes from the mid-19th century, when the practice of beating horses to make them go faster was often viewed as acceptable. To beat a dead horse would be pointless, as it wouldn't be able to go anywhere. An early written form of this popular idiom is found in an 1859 issue of the London paper Watchman and Wesleyan Advertiser: “It was notorious that Mr. Bright was dissatisfied with his winter reform campaign and rumor said that he had given up his effort with the exclamation that it was like flogging a dead horse.”