This article explains when you can delete inventory order requests and items in an inventory order request.
An Inventory Order Request can only be deleted if it's status is 'Nothing Ordered'. Once you order at least one of the items in a request it's status changes to 'Pending'. At this point it cannot be deleted. The reason for this is because once you order an item the 'on order' amount for the inventory item changes. If you delete the item on the request then you will never be able to receive the amount ordered an the 'on order' amount will never decrease.
Items on an Inventory Order Request can only be deleted if they have not been ordered, that is, their status is 'Nothing Ordered'. This is for the same reason as above. You can also not delete an item if it's the only item in a request.