Illegal blocking techniques

[BigGref]

I was just looking through the rules book in the blocking techniques part and thought of a question. Where do we as officials draw the line with a player using his forearm to block instead of his hands? it says that the player's elbow should not be in advance of the hands, when we see this type of "Shaq Defense" blocking where do we draw the line? Should we ask them to try not to block that way or just let it go, or just flag them on the spot?

QCA (questions comments answers)