What do i do when someone takes my gift, but doesn't want to acknowledge it on steam gifts?

Wait. They have 7 days to do so.

wat?

Happened something like that but on reverse. I gifted a game (actually was a regift, I was kind of n00b by then, already got suspended by that), send it to the winner, he said that the key was invalid. I said him that I was sorry and that I would send him other game. He.didn't answer, but he marked it as recieved and was on his library. Don't know what happened.

1: Wait until seven (7) days have passed since the giveaway completed.
2: If the game is still not marked as received, file a support ticket.