In doubt, you should read the FAQ. - http://www.steamgifts.com/about/faq

Here's what you need to do:
"I accidentally created a giveaway, or the gift is no longer available, how do I delete my giveaway?

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."