Who's Who message indicator for Case Management and PO Approval Workflow

I have a user who is an assignee in Case Management, who also is a PO approver. Case Management workflow for email notifications is turned off, but user still receives the notification due to who's who message indicator = 1. User does not want the case email notification. If I set the message indicator = 0, user won't get the case email notification, but won't get PO approval notification either. Is that a correct statement? Is there a way to turn off only the case management email notification only but leave the PO approval notification in tact? Thanks in advance