Beating a dead horse
In zh-yue 鞭打死馬 (bin1 daa2 sei2 maa5)
This idiom is a direct translation, emphasizing the futility of continuing an action.
Example
"再爭論落去都冇意思，就好似鞭打死馬咁。(Zoi3 zang1 leon6 lok6 heoi3 dou1 mou5 ji3 si1, zau6 hou2 ci5 bin1 daa2 sei2 maa5 gam2.)"
