To delete item lines from an ASN, make sure the item is selected. Note: You must be in the Item level. Notice how Item is highlighted in green in the example below). Select Delete in the Action Bar and choose Delete Selected Item.
If the item is the only item in the carton (pack), it will still show up red. This means you'll need to delete the carton as well. From the pack level (notice how Pack is now green as it is the selected level), select the carton (pack) that needs to be deleted. Click Delete in the Action Bar and select Delete Selected Pack.