BADIs to validate PO Approval in My Inbox App

We person a request wherever we request to power definite Substitute users from approving POs erstwhile they click connected 'Approve' fastener successful My Inbox App. We person explored antithetic options from stopping the Work point from being received by the Substitute to searching for BADIs which trigger connected click of 'Approve' fastener but person not been fruitful successful our analysis. Hence, would request immoderate assistance successful this requirement. 

We person flexible WF (WS00800238) configured for Purchase Orders successful our system. The request is to halt circumstantial substitute users from approving a PO (if they hap to beryllium the creator/requestor of the PO). Our attack was to place BADIs which would trigger connected click of 'Approve' successful My Inbox app truthful we tin validate arsenic per our request and contented an mistake connection to halt the idiosyncratic from successfully approving. Kindly fto america cognize however this tin beryllium achieved (without having to widen the My Inbox App to a customized one)?

Thank you.