Yes.

There are certain conditions that allow for a reroll. For example, if you can't contact the winner within 7 days, despite sending him messages over Steam, e-mailing him, and/or replying to his comments on the website (as any reply will leave a notification), you can request for a new winner. After you make your request, the support staff checks your request and approves it if conditions are met (for the example above, you need to provide proof that you have attempted contact, like screenshots of sent comments or e-mails).

In this particular case, I have checked the winners for any site infractions. I've reported my findings to support, and after their inspection, they've suspended one of the winners (temporarily) for breaking site rules. Since suspended users can't use the site, it's considered a valid reason for reroll.

You can check what other reasons are acceptable by going to the Winners page (on the left hand side) of any of your giveaways, and clicking on "Request New Winner". It'll direct you to a request form, with a drop down menu of reasons.