Regifter

you won it its yours

Activate it and report him. It is still your win.

Hmm.... Tough one. You activate it, but you can tell the person the won gift must be activated. The person might provide you with a second copy or buy a second copy him/herself. If he/she activates it him/herself, you mark it as not received.

I don't think there are any rules around this. Generally people seem to think it best for the winner to activate it so that the original gifter gets their gift to someone decent in the end. I've seen original gifters actually wait til the regifter's winner activates and marks as received to report them. Never heard of the winner's side of it though... so up to you I guess..

Regift it again to show dominance

Hmm, that's a pretty good question.
First of all, you should definitely report them, since they apparently have broken a rule. After that, I'm not sure but I assume you may activate the game, since what happens to the game is not that important at this point. You can't really give it back to the original giveaway creator since they have no proof it hasn't been activated yet and thus cannot request a reroll.

However, if it's a Steam gift rather than a key, you should probably proceed otherwise.

EVERYTHING IS AWESOOOOOOMEE!!

Two days ago I was on the other end of such situation. And the person recieving the regift was surely disappointed. It was a HB link, someone won it from me and instantly made a giveaway with the same game without activating what he won from me. I promplty gave the HB link to a friend that was online. He activated the game, and regifter's winner got a used link.

Activate but don't report him. Dick move if you are taking what he's giving. Let someone else report.