Hi,
Do you want system to not to create network on level 1 and level 2 wbs. Have I understood this correctly.?
Just think of a situation, if you do not set account assignment element indicator on those 2 levels of WBS then would you be able to create Network.? Try this in your development/quality client. If it works then check you should not have maintained the tick mark on " All Acct. Assgin Element" in OPSA (Project Profile). Remove that if you have maintained.
Now, Maintain validation/substitution so that it works like whenever system encounters PRPS-STUFE = 1 OR 2 then it should not select PRPS-BELKZ.
My idea of providing this logic is simple " No Network means->No Mat Component Assignment->No Direct PR".
Else, you can also maintain User Status but you need to search for correct business transaction and I also thought of changing the Procurement Parameter to NEVER in Res/Pur. Req. The same you can opt in OPUU (Network profile) but in this case you need to create network so its of no use. Try Implementing my first logic and let me know if it helps you.
I hope this may help you.
Regards,
Amit