Well, if you want to delete the giveaway instead, you'd have to ask the winner for consent in order for us to approve the request. Just like you would have to ask the new winner, if it turns out the key doesn't work. But even then, you wouldn't know that your key ever worked, given that even keys from reputable stores regularly are already either revoked by the developer or activated on another account - especially Humble Bundle keys are known to "expire" after a certain amount of time. I likely lost about 100 games to HB myself - games that I payed for.