>It does make me wonder how a warplane stops a merchant vessel without blowing it up if the radio doesn’t work

We saw how from the Houthis and US military: You send a helicopter with a few dudes with guns. Marine vessels are unarmed, including the people on board. They can't fight off or run from the helicopter.

If for whatever reason that's not an option, you shoot it with the 5inch gun on a destroyer. Maybe a warning shot across the bow first. Maybe you literally ram it with the destroyer if you are feeling weird, as China and Venezuela have done. Awkwardly, when Venezuela did that, they rammed a vessel that just so happens to be reinforced for ice breaking, so the warship was damaged and the cruise ship was not really.