Script (guión): Idioms, ‘beating a dead horse’. If someone is trying to convince people to do something without any hope of succeeding then we say they are beating a dead horse. For example, if I'm trying to convince my family to come skiing on a skiing holiday, I may say, ‘Come on, it will be fun. Lots of slippery snow and mountains and there will be snow’ and nobody looks interested. I might still continue to try. ‘Come on Grandma, you love snow.’
But still nothing's happening you could say I'm beating a dead horse. There is no hope.