A Barney Fife
To Barney Fife something means to stop it before it gets too crazy, like ending a fight before it turns into a bloodbath.
I Barney Fived my little brother before he could throw his entire lunch at me.
She Barney Fived the argument before it turned into a screaming match.
He Barney Fived the prank before it became a full-blown prank war.