Avoid creating PR and PO while creating the sales order

Hello Folks,

I have a requirement to stop creating the PR and PO if Cost center is blank.

i have used the user exit SAVE_DOC_PREPARE and there i am updating the confirm quantity = 0(From Schedule lines ) if cost center KOSTL is blank. but stil PR and PO is getting generated.

could you please suggest me how to stop creating PR and PO while saving the sales order.

Thanks in Advance!!