Cry wolf

The politicians would cry wolf at the slightest provocation so when the real threat appeared no one believed them.

The newspaper placards that had cried "wolf!" so often, cried "wolf!" now in vain.

[…]and the critical sense of the professors counts for little, for they cry wolf too often[…]

[…]those who created the worst economic mess in postwar history should be the last people crying wolf 1,000 days into this administration[…]

These intensifying disasters now striking richer countries, she said, show that developing countries seeking the world’s help to fight climate change “have not been crying wolf.”