In the imperative sentence "Shut up, Ding", "Ding" is the subject, and is being instructed - however futilely - to cease his havering.
Had you written "Shut up Ding", "Ding" would more naturally be construed as the object, and the unknown subject would be our hypothetical assassin.
Remind me never to ask you to feed a panda. Lynne Truss is a cunt.