Hi Anita,
Implement the method BBP_WF_LIST in the BADI BBP_WF_LIST.
Use the filter type BUS2201 and restrict the list by filtering the field CREATED_BY in the header of the PO, so that your requirement is achieved.
For more information, and for the sample code refer this link.
http://wiki.sdn.sap.com/wiki/display/SRM/Restricting+Search+criteira+results+in+SRM
Hope this helps.
Regards,
Karthik Babu