To prevent the PO creation, you can put a hard stop error in the same window (SoCo wizard step 3). To do this, you can create an enhancement on the UPDATE method of the mapper class.
In method /SAPSRM/IF_CLL_MAPPER~UPDATE of interface /SAPSRM/CL_CH_WD_DOM_SOCO_GAF3
SAMPLE CODE:
<insert validation logic here>
....
*add error message
CALL METHOD mo_cll_message_handler->add_validation_error
EXPORTING
iv_msgno =
iv_msgid =
iv_msgv1 = .
This should display an error on the message window to prevent the user from proceeding to the next step/window. This should fit your requirement.
Let me know should there be any questions.
Regards,
Justine