To be able to sell certain items only at the POS without having to offer them in your online shop, these items must not be available in your main shop. To do this, open den item details of the respective item(s), navigate to the Categories tab and remove the main shop category in the Assigned categories section. If the item is no longer assigned a category, this is not a problem as the item is still available for purchase via the POS app. Categories can be created and edited under Items → Categories.
Alternatively, you can deactivate the item by unchecking the box Active in the Basic info tab. In the POS app, this item will then be displayed as deactivated, but can be sold as usual.