Hi Zena,
As far as I know, there is no such functionalities in Shopping Cart to control whether or not to output its follow-on document (PO).
If you don't want to issue PO output at all at any stage, you may consider to set the action entry in the customizing table for PO (BUS2201) as inactive.
BBPC_ACTIONMAP
BUS2201 Output CL_BBP_ACTION_OUTPUT_PO
Regards,
Wendy