This happened to me with TripIt. A colleague invited me and they started sending me spam without any action from myself, and it was to my corporate account without an easy spam button like yahoo mail or gmail. I couldn't unsubscribe without creating an account first. I ended up emailing their corporate address notifying them they were violating CAN-SPAM and they manually removed me, and a PR contact apologised. I advised them they should give non-users a way to unsubscribe. I'm not sure if they ended up following my advice. My colleagues are loyal users, but just because your users love you doesn't mean your non-users want your email.