Save your breath
Definition 1 of 1
Pro Tip 1/3Use Sparingly
Use this phrase only when it's clear that further discussion is pointless. 
She's made up her mind on the topic, you can save your breath trying to convince her otherwise.
Pro Tip 2/3Implied Frustration
Signals speaker's annoyance or frustration regarding the stubbornness or closed-mindedness of others. 
I've told him a hundred times already; save your breath, he just doesn't care.
Pro Tip 3/3Preemptive Advice
Often used to advise someone before they waste effort in an argument or explanation. 
They've already decided on the proposal—save your breath with the presentation.