The whole point of Kiai is, as far as i know, to be as distracting and play disrupting as possible and not be able to be disabled.
I don't know why. Maybe it's to prevent the game from getting popular to the point where Peppy gets swamped with DMCA's for all the music on the site, maybe it's because it is integral to the feel of the game [lol], but whatever it is, i would sure like for at least things like this to be changed.
The reason I think it has some sort of deep seeded purpose is because there's no other rational reason why it wouldn't be able to be turned off like everything else visual in the game that affects gameplay, either through skinning with the famous 1px by 1px images that simplify nearly anything you don't need/want, or by a simple checkbox.