(put) a nail in the coffin of, a final nail in the coffin

(a) nail in the coffin of something

an action that will cause something to end 

This report on the effects of smoking is another nail in the 

coffin of the tobacco industry. 

We thought the firings would put the final nail 

in the union's coffin, but in fact, 

the union has grown in size and 

importance.

a final nail in the coffin

an event that causes the failure of something that had already started to fail:
Each successive  revelation of incompetence is another nail in the chairman's coffin.
That report  drove the final nail in the company's coffin.