This goes against the typical Roman military policy of near-constant Rape, Pillage, and Burn, but though it's not uncommon in Westeros, it's still relatively frowned upon (except by the Ironborn, but nobody likes them), so Octavian, ever the pragmatist, apparently decided to limit the bloodshed.
He was given a rather colorful punishment in the form of a standard flogging and then a maester was summoned to educate him on the proper usage of the Westerosi language.