beat a dead horse

To continue to focus on something—especially an issue or topic—that is no longer of any use or relevance. We've all moved on from that problem, so there's no use beating a dead horse.

Also, flog a dead horse. Try to revive interest in a hopeless issue. For example, Politicians who favor the old single-tax idea are beating a dead horse. From the 1600s on the term dead horse was used figuratively to mean "something of no current value," specifically an advance in pay or other debt that had to be worked ("flogged") off. [Second half of 1800s]

To pursue a futile goal or belabor a point to no end. That this sort of behavior makes no sense was pointed out by the Roman playwright Plautus in 195 b.c. The analogy certainly seems ludicrous; what coachman or driver would actually take a whip to a dead animal? The figurative meaning has been applied for centuries as well; often it is used in politics, concerning an issue that is of little interest to voters. However, some writers, John Ciardi among them, cite a quite different source for the cliché. In the late eighteenth century, British merchant seamen often were paid in advance, at the time they were hired. Many would spend this sum, called a dead horse, before the ship sailed. They then could draw no more pay until they had worked off the amount of the advance, or until “the dead horse was flogged.”