Sent a key and the user marked it as not received, so it ruins my feedback, what to do?

Haven't been on steamgifts in a while (had to reset cookies on browser and was too lazy to link steam account again), and came back to see that I had only 80% recieved, and saw that the last giveaway I did, the user said it was not received after I had sent it.

Checked my email to make sure I did actually send it, and checked my Steam account to make sure I didn't accidentally redeem it, and I had sent it and hadn't redeemed it. So they've taken the key and then given bad feedback, so was wondering how I can fix this?

Printscreened the e-mail, date clearly shown, key was most definitely not redeemed.

http://tinypic.com/r/30mn3lt/5

Thanks in advance.