While the MK-48 torpedo is positively a *mother*, what we need here is something more discreet. It doesn’t even have to be too high tech.
Someone suggested putting a very large concrete block on a surface ship, with heavy cables through the block. Then as the Nork boat passes nearby disable its screws with heavy fishing net connected to those cables. Then drop the concrete block overboard, dragging the sub due south, butt first.
The only sound that is heard on sonar is the pop when it hits crush depth. And even if someone decides to inspect the wreck, all they will find is fishing net.