With textiles this is usually a hole punch or something with the tag. With hardware we had the serial number recorded.
But consumers don’t care. If they buy something from a vendor they think is selling them something as new and the vendor tells them to go the manufacturer, the customer doesn’t care that you marked it as not eligible for warranty. They just want that coverage
We even had customers write ragebait Reddit posts claiming we were unfairly denying warranties, people sending stories to popular newsletters and journalists, and other attempts to make us look bad for not honoring warranties on products they bought through gray market channels.