Hi,
After trying Ankur procedure also , if u are not able to delete that PR , means its shaving some in consistency, to rectify that RUN the programme in SE38 , ( RLORD035) for deletion of PR & PO .And run CJEN & CJBN after deletion of the PR , to the Project WBS.
This programme will deletes only the wrong PR & PO only .
I hope this might be useful to your problem.
Regards,
Rajesh.