Hi Virendra,
I can suggest you other way round solution.
If you are sure that those two WBS will not take any actual or commitment cost, then its better to NOT to mark those WBS elements as Account Assignment Element. I mean, if you do not set the indicator of Acct. Assignment Element (Untick Acct Assignment Element) on those WBS, then user will not be able to create Networks on those WBS Elements. Try this on your development or quality client and let me know if it works.
If it works, then you just need to disabled the Acct. Assignment Element indicator for those two WBS element and you can achieve that through substitution and field selection of WBS as well.
OR
Just look at OPSG settings at tab component. Just look if that can solve your purpose because your requirement is to stop PR from those WBS element. (I am just wondering if you can stop user from creating/change material components from those WBS-Networks)
OR
Go for User Status and set it to on those WBS Elements. Object Types will be Networks and Network Header.
OR
Try to achieve a validation or substitution where in you can mark Res./Pur. Req as 1 (Never) only for those two WBS elements. In this way user cannot create PR on those two WBS elements.
Regards,
Amit