Restrict WBS elements while creating Requesitions

Hi Experts,

We have a security requirement to restrict users from using different WBS elements while creating Purchase Requisitions ME51N. I tried to use auth objects C_PROJ_VNR and C_PRPS_VNR by using field Person Responsible but still user is able to use other WBS elements and create PR's.

Please advise how we should acheive this restriction at WBS element level so user should be able to create PR for only that particular WBS level not for all WBS ?

I noticed one thing, ME51N doesn't check for authorization objects for WBS elements like C_PROJ_VNR and C_PRPS_VNR. How should we activate these authorization checks ?

Regards,

Salman