Barcode Scanning still functions when the "Inventory & Purchase Orders" preference is disabled which should also disable the Advanced Inventory settings that contains the barcode scanning feature.
Using the Advanced Inventory Barcode Wizard feature creates barcodes for the item list items you designate. If barcodes were created for such items as service items and non-inventory part items, in order to accommodate these other item types, the barcode features will still work for those other items even when the "Inventory and purchase orders are active" preference is disabled.
The user will need to disable the barcode preference in order to stop the barcode scanning feature. They can change the preference as follows:
To enable/disable the barcode preference:
This will reset the preferences to what you need.