How can we hide or disable the "Add Lines" button during PO invoice creation in the Supplier Portal?

When suppliers are creating invoices for Purchase Orders in the Supplier Portal, we would like to prevent them from manually adding new invoice lines by hiding or disabling the "Add Lines" (+) button (see highlighted example below). We have explored standard role/permission settings as well as sandbox and page personalization tools, but could not find an option to remove or restrict this button's visibility.

Is there a supported configuration approach, workaround, or enhancement planned to allow the hiding or disabling of this button on the supplier-facing invoice creation page?

Reference Screenshot: