You have to contact the winner or ask the winner to contact you within 7 days.

Better you contact them and send them their steam gift or steam key for the game. This way you avoid people pretending to be winners

I think someone answered your query well in the discussion you started.

You can't give away games from your steam library, you need to have a spare copy or unused key for the game (so if you don't have such copy you better close this giveaway yourself). When the time of your giveaway is out there will be paragraph "winner" in the left section of this page. There will be email of randomly chosen member of this site on witch you should send your gift copy or key of the game. That's the simple version of this site concept, but you should read FAQ for more detailed explanation and to avoid further misunderstandings.
I hope I've been some help for you. Welcome to the site and have a good day!

it won't disappeared from your library since you can't give or supress anything (except free games) from your steam library. so to be able to give a steam games, you must have an unused steam key to give, or a steam gift (wich mean you will buy another alan wakes's american nightmare to give it)

If you think you won't be able to give this games to the winner, then it is better to cancel this giveaway, otherwise you will have only "gift unsent" wich will be bad for your reputation on this site^^

Hope i could answer your questoin