Hi,
You may try with approval procedure to restrict user A to add item group 'Spare'.
Assign below query in approval template and try to add PO.
SELECT distinct 'true' FROM OPOR T0 INNER JOIN POR1 T1 ON T0.DocEntry = T1.DocEntry INNER JOIN OUSR T2 ON T0.UserSign = T2.USERID INNER JOIN OITM T3 ON T1.ItemCode = T3.ItemCode WHERE T3.[ItemCode] = $[$38.1.0] and T3.[ItmsGrpCod] = '112' and t2.userid = '11'
Note:
1. Replace 112 and 11 with your group code and user code.
2. The above procedure will block PO, if first line the PO meets approval query.
Same way you can apply for User B for item group "purchase"
Thanks & Regards,
Nagarajan