Repercussions for refusing to send a game and marked Not Received

What Happened? Why did they or you not want to send the giveaway?

You get a suspension for not sending a win IF you do it with the intention to prevent that someone get his win, without that he broke a rule.
It's something different when you have a faulty key and can't replace it etc.. In such a case you don't get a suspension on top to the lost 1 GA slot and the stat change (+1 not sent game) in the profile.

Withholding a win is not cool.
Having a faulty key and unable (or reasonably unwilling- massive sale, or bundle ended) to replace is tolerated, accidents happen.
I'm fairly sure the mods keep note of these events, either actively, or when they notice it being too common.

Nothing of this is quantified, it's mostly on expecting good will and honesty, as my experience goes here.