www.ditl.org

My response was invariably, "Where were you when your kid bought that CD?"

Nice one.
How do they usualy respond to that?

Nine times out of ten, they sort of bowed their heads a little bit and left.

At least they aknowleged their stupidity.

I have always found that a simple, quiet, factual yet non-judgemental response is most effective at shutting up the drek-heads who come at you screaming, blithering, and spewing fire and brimstone. Generally, those types are already trying to compensate with volume for lack of rationality, and then they feel awkward yelling and screaming when you won't in return.

Your job seems rather similar to mine in some ways.

You'd be surprised how vocal some parents can get when I call them in to discuss their kid's lack of behaviour.

I wouldn't be surprised. My wife was an elementary teacher, and she is now a literacy coach - so she gets the same reactions from other teachers whose lessons or differentiated instruction she is trying to improve.