flog a dead horse

flog a dead horse AND beat a dead horse (to waste time trying to do something that will not succeed; to continue to pursue a cause that has no hope of success; to dwell tiresomely on a matter that has already been decided) — ~ (как) мертвому припарка; ~ решетом воду носить;

Example 1: You're flogging a dead horse trying to persuade Simon to come to Spain with us - he hates going abroad. (CIDI)

Example 2: Do you think it's worth sending my manuscript to other publishers or I am just beating a dead horse?(CIDI)

‘Beating a dead horse’ is also talking and talking and talking about a subject that has been talked about for a long time. You are getting nowhere. {often used in the negative, as in stop beating a dead horse.} — переливать из пустого в порожнее

Example: We’ve talked about how he misbehaved at the party for a week now. We need to stop beating a dead horse. (David W. Miller )— ...Хватит уже переливать из пустого в порожнее

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посл. голую овцу стричь; мёртвого лечить (дословно: Стегать мёртвую лошадь, т. е. заниматься явно бесполезным делом) {Milanya}

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махать кулаками после

махать кулаками после драки
после драки кулаками не машут — stop beating a dead horse