How to prevent deleting or changing Attachments while approving invoices

When AP invoices sent for approval with attachments,approvers can view and modify attachment currently. With or without May be changed checked, while attaching the File to an invoice, Approver could delete. But we require to stop the ability for the approver to delete the invoice. Page personalization, is not available for the Delete and update button in the Attachment page browsed through Notifications.

Please help asap with a setup, or functionality or any solution to stop attachment being deleted.

We have to restrict this option , to NOT TO ALLOW approvers to delete, to our specific OUs.

Your earliest response is appreciated.