To put the ____ on something is to end it (6)
I believe the answer is:
kibosh
(Other definitions for kibosh that I've seen before include "In slang, put the . . . . . . on - end it" , "Hibs O.K. (anag) - put a stop to" , "Terminate" , "Put the mockers on" , "Put this on something and stop it from happening" .)