If the business process requires the Purchaser on a PO to be a different person to the Approver of that PO, then set the Purchasing Control Parameter - 'PO Purchaser/Approver different' to Yes.
With this set, the Approver on any PO must be a different person to the Purchaser on that PO. When selecting the Approver, if there is a conflict a warning 'Reselect - the Approver must be different to the Purchaser' is displayed.
Note that this is an error after approval selection not a filter on the Approval list.