Which 'law' demands Gaza be turned into an open air prison and its citizens be metered out collective punishment Keith?

Under the United Nations charter states have the right to self-defense, (jus ad bellum) but defensive actions should not exceed necessity.
States may deter future assaults aggressively, but they're not supposed to launch a counterattack that's radically incommensurate with the actual threat.

International humanitarian law takes into account the legitimate military imperatives of warring parties.
There is no justification for perpetrating serious violations of IHL.