There's a "Delete Giveaway" in the menu to the left.

Now you cannot anymore. You waited to long.

Now you have 4 options:

1. Ask the winner for a permission to delete this giveaway.
2. Buy the game and send him.
3. Wait 7 days and receive your "Not received".
4. Hope the winner is stupid and he activate the game, give you a "Received" and only he will get the suspend.