To beat a dead horse means to continue with a topic / matter that is already finished and done with. It refers to the fact that it is absurd and stupid to keep flogging a horse to get up and move when it is dead.
1. The corruption charges against the politician had been cleared a year ago, but yet the opposing candidate insisted on beating a dead horse by raising the issue during campaigning.
2. Shrinivas hated it when his cousin continued to beat a dead horse in discussing the Goa family property matter.
3. The basketball player was fed up of the coach constantly beating a dead horse in talking about this foot faults; the player felt he had improved and there was no need to raise the point again.