Override Email address or stop email notification for some users

We have a PO Requisition workflow which sends email notification to end user for approval. Upon receiving the notification, end user takes action through email. We have requirement to stop email notifications to all end users but workflow notification should continue to flow. Can anyone let me know how to achieve this ? Do we have to do any change at workflow level or OS level ?

Also, is it possible to divert all emails for PO requisition to specific email address instead. For example, if user's email address is ab@cd.com then any email notification toab@cd.com or any other email address should go to