"Knock it off" has been part of the Joint-Service brevity comm at least since 1998. It may have been before that, but that's the first time I became aware of it officially.
Just as an aside, if you look at the original script for Top Gun from the early 1980s, "KIO" is in there, too.
A little snippet of that remains in the actual movie comm, although perverted from its original context, when Viper says something to the effect of, "Knock it off gentlemen....." during the dogfight in which Maverick floor-kills himself.