What happens if you do not redeem a key you won?

you get a suspension.
edit: and on several Blacklists

edit2: if you don't want the game you can ask the GA maker to ask for a reroll, so someone else, who really wants the game would get it.
but that is only assuming you didn't activate it on another account

If you click received and get reported, then suspension, and later unable to join certain SG tool giveaways.