How to PM a winner here?

Hey, so there was a problem with one of my giveaways and I had to have it re-rolled with another winner. In addition to that, the key for it turned out invalid - but I have a Steam gift of the game that I intend to give to the winner instead. Unfortunately the new winner isn't reading his E-Mail and he blocks me on Steam whenever I try to add him. Now he marked the game as non received because he doesn't realize that instead of the Key I have a Gift. I've removed the key since but it's probably too late for him to notice or make sense of it.

What I want to do is get in touch with him through the message system here to let him know that he needs to accept me on Steam or add me himself so that I can send him the gift. Surely he'd see that. Unfortunately I can't find where I do that, is it even possible?

Please let me know. Thanks guys!