There is a plethora of expressions you could use for this, but I've compiled a list of phrases that I've heard expressions likewhich I don't think anyone else has put in their answers. Here are a few of them...
- spitting in a lake/ocean
- emptying the ocean
- digging to China
- convincing a woman
- spinning your wheels
- talking to a wall
- sweeping a dirt floor
- spitting in the desert
- listening to a rock
- burying a house
- kicking water
- putting the kettle in the fridge