Hi, Is it true that we can't cancel an order through Suite Script ?

You can close an order but as far as I know the only (unsupported) way to cancel via script is to call the url that the browser navigates to when you click cancel in the UI

You can set the quantity of each item to 0

You can use this undocumented SuiteScript 1.0 function which is what NetSuite calls internally when you cancel an order cancel_approve_order(<record_id>, 'cancel');