How do you stop fake returns?

Returns- I have to 5 returns that the customer claimed that the product did NOT work- after receiving the product we noticed that the customer did NOT use the product (did NOT even open it) . How did the Product NOT work if the customer did NOT even open it? My concern is that after so many returns Amazon starts penalizing the vendor, and I am being charged for a return label when there was nothing wrong with the product. How do I resolve this issue. Amazon automatically refunds them when the product is scanned and I loos money on return labels when there was nothing wrong with the product. Is there somewhere on amazon were you can disputes these returns?