Here's a tale from Matt Blumberg, CEO of ReturnPath, on how confirmed opt-in, aka double opt-in, isn't necessarily enough to 100% prevent spam complaints. Why? Because there's no fool proof guard against stupid. The stupid, in this case, comes from the recipient. The recipient who signed up, CONFIRMED, then went on a rampage of idiocy hassling Matt's wife and making threats. Over mail he signed up for, with confirmed opt-in.
Matt kindly decides against outing the waste of space responsible. Which is a shame, as they deserve to be outed.
Oddly enough, this reminds me of my days back at the Artists' Quarter in St. Paul, MN. Occasionally we'd have a patron who would go off the rails. Decide they don't like the music, or the guy next to them, or the phase of the moon. They'd start inappropriately shouting, yelling, poking at people around them, the bartnder, waitresses, door man. On the few occasions that I observed this, my solution was to physically eject that patron from the club, at whatever level of effort it took. In my estimation, this was the right solution. If you're an idiot, you forfeit your right to hang with us, and it's not inappropriate for me to push you out of the circle.
Sadly, it's probably not possible for ReturnPath to force this guy off of the internet. But if I were Matt, I'd probably be sure this guy never received a piece of ReturnPath-related email ever again, no matter how he signs up or verifies consent.