Hi,
You can use two BADIs to achieve your requirement ME_PROCESS_PO_CUST. and ME_HOLD_PO
ME_PROCESS_PO_CUST. Whenever a Customer is Creating STO (Document Type), Define here Supplying plant you want to restrict and add custom message so that system should throw an error.
ME_HOLD_PO can be used to write logic for your restriction to create STO, so that it will not allow to save the PO,
But rather going for development I will suggest to check what
advised in previous post, it will work, which is easiest way,