SWWLTBO Tells me that you MUST NEVER tell the child you are disappointed in him/her ~ only that their behaviour disappoints you. That way the child is left feeling positive about itself and, hopefully, will question its own behaviour.
She also tells me (as gleaned from her recently completed PGCE) that this is, in her considered opinion as a mother of three, total bollox and a clip round the ear is quicker and simpler all round, it is also clearly understood by the child who doesn't actually possess the reasoning skills to work out what it is about their behaviour that is disappointing.