Flog a dead horse
Definition 1 of 1
Pro Tip 1/3Wasted Effort
The phrase implies that any effort put into the task is wasted and pointless. 
He's still fixing that old car, but it's like flogging a dead horse.
Pro Tip 2/3Futile Argument
Often used to suggest that arguing or persuading is useless because the decision is already made. 
Don't bother arguing with her about the trip; it's flogging a dead horse.
Pro Tip 3/3Move On
Suggests that it's time to stop an action and proceed to something else more productive. 
Stop revising the proposal, it's just flogging a dead horse now.