When my dad saw me picking my nose, he'd say: "Are you going fishing?"
Me, innocently: "Huh? No. Why?"
Him: "I see you're digging for bait."

I probably only fell for that joke once, and after that, "Are you going fishing?" was a sufficient reprimand to get me to stop.

I also used it on my own kid. It works.

Dad also liked non-sequiturs like:

"How big would you be if you was twice as big as half?"

In the realm of reprimands and threats:

You're gettin' too big for your britches.

I'll take you down a notch.

I'll jerk a knot in your tail.