Giveaway ended a week ago, person hasn't clicked "Recieved"

Tried to contact him on Steam?

Sent an assassin?

If you've already sent the winner a key code through email I highly doubt the support staff will give you a rereoll.

Check the persons account to see if they used the key or not before doing anything.

Tried adding him/her on steam and asking to update the received status? I can be tiresome but you might have to do it :(
Had an issue like that and even after telling the gift receiver multiple times, he wouldn't update the received status.
Had to contact support after that to manually adjust received status.

Keep the contacting support step as a last resort as they are getting more than enough support tickets per day :P

1) Check their profile to see if they have the game.
2) Send them a friend request/comment on their profile.
3) ??? Dunno

Send a tank after him, and oh yeah, support. Good luck.

4) Track 'Em, Find 'Em, Kill 'Em

Check their steam profile. If they didn't activate it, maybe they never got your message. This is why I prefer sending keys via steam chat and screen shot. Though I give mine a high contrib so I can avoid retards.

This is why you don't wonna send keys via email

perhaps he just does not know it. i did not know it, too in my first win until the person told me in the chat

Learn from it also while trying to solve it.

EDIT: The solving part is to contact him and see what he did.

You should look at his steam account, if it's public, and see if the game has appeared there. That'll at least give you a bit more information (you can also check if he's been active on here.) Also, has he received a gift before? If not, he might not understand that he has to confirm it.

It's unlikely that he's trying to pull something (if he were, after a week, he'd have reported it as not-received.) Most likely there's just some misunderstanding, so be polite.

Stalk him.