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So don't enter them to begin with? Or decline the win and let it be rerolled.

Welcome to Steamgifts.
As it seems you have not done so yet, may I strongly suggest you read the FAQ & guidelines. It is also a good idea to keep track of the announcements done by Steamgifts' admin.

In case you are unsure about the rules stated in the FAQ & guidelines, use the search function of the discussion board, as most questions have been asked plenty of time.

Enjoy your stay at Steamgifts & good luck!

From the FAQ:

Can I enter a giveaway for a friend?
You should only enter to win giveaways for yourself, and if you win a game, it must be activated and redeemed to the Steam account you used during registration. If a friend is interesting in joining giveaways, they'll need to register for their own account.

and

If I win a game, can I regift, trade, or sell it?
According to our guidelines, no, all games you win must be activated and redeemed on the Steam account you used during registration. Failure to activate games you have received will result in a suspension.

In other words: it's so against the rules, just don't ever do it. The first few times you will receive a suspension when caught, in the long run this might lead to a permanent ban from this site.

Edit: best way to go is to only enter for games you really want. This way this site is most fun.