The phrase "dead horse" means to keep discussing or working on something that has already been resolved or is no longer useful, resulting in wasting time and effort. Full definition
I may not be vegan, but that doesn't mean I like beating dead horses. (deniseminger.com)
Yes, it may sound like we're beating the online dating dead horse here, but we want to make sure that you're taking this all seriously. (ourbestonlinedatingsites.com)
This was 3 years ago and they are still beating that dead horse in all of their marketing material. (goodereader.com)