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.
Regards,
Amit