From the site's FAQ:

When viewing your giveaway, you'll see a button with the name Delete Giveaway in the left column. Clicking this button will let you specify a reason for the deletion, and it'll walk you through the process of creating a support ticket. If the giveaway is still open, the ticket will be automatically approved and the giveaway removed. If the giveaway has already ended, then the ticket will need to be approved by a member of support.

So, if you intend to delete a giveaway, you can do it before it ends with more or less no harm done. But if you want to do it after it ends, my understanding is that you'll need to have a valid reason and I dont think "I'm not sure the key works" is valid enough. Again, I dont have any official confirmation, I'm just going by the FAQ and my past experiences on this site. My guess is, if the keys dont work and the winner doesn't agree to the giveaways deletion, it might not go all too well.