Disable opt-in e-mails for joining groups
You probably saw this but there is a second setting that deals with Double Opt-In - but since you only referred to one, the other one is 


  Enable Double Opt-in for Profiles which use the "Add to Group" setting 
  When CiviMail is enabled and a profile uses the "Add to Group" setting, users who complete the profile form will receive a confirmation email. They must respond (opt-in) before they are added to the group.


Also at this path - civicrm/admin/setting/preferences/mailing?reset=1