Winner of my giveaway not responding to me

After another day, you can ask for a reroll. Try sending them a reminder email in the meantime, but a Steam invite and the game sent to email should be warning enough for them.

1. Wait for 7 days
2. Cancel gift (so he cannot redeem it after you reroll)
3. Reroll (provide proof of attempt to contact winner)

The submitter and winner(s) of the giveaway have seven (7) days to complete the gifting process. This includes the activation of the gift on the Steam account of the winner.

Just wait one more day, then cancel the gift and reroll :)

Click on request a new winner :D

Funny people those winners are. The winner of one of my giveaways doesn't want to take his prize from my inventory as well. Although not only he's been on-line on both Steam and Steam Gifts, he even marked gift as received.