Infelizmente, o CSS não pode ser "desativado" ou revertido depois de ativado:
::-webkit-scrollbar cannot be simply overridden to get the default style, the only way to do it is to remove all ::-webkit-scrollbar rules from the code. At that point, scrollable areas have to be forced to redraw the scrollbars. To do that you either quickly add and remove display:none; from or do the same thing with overflow:hidden; on scrollable elements. The problem with the first one is that the page flashes white at every page load; the second one is resource-intensive as it would have to whether check any element on the page overflows—not ideal.
O link acima é de um script que removerá completamente as barras personalizadas, deixando você sem barra de rolagem.