Using ShipStation's Branded Returns Portal allows your customers to conveniently return items without the delay of waiting for you to first authorize the return. This is made effective by ShipStation's Returnable product option. This is an item-level option that you can find on the Product Default. By default, this setting will be enabled.
If you have certain products you don't want your customers to return, or you'd prefer further feedback on why the customer is returning the item, etc. prior to authorizing the return, simply deselect the box on the product default. Or you can update this field in bulk via importing an updated Product CSV (A sample of the CSV template can be downloaded here).
When a customer tries to print a return label for a non-returnable item via the Returns portal, they'll receive an error "This item is unable to be returned":