Yeah. imsuperweird is right. You log in to the COD app. Then, select Clan Wars option and select the Diamond Division Invite which you can see in the menu button at the upper left of the screen. You can see an option there to opt out. By the way, only the clan leader can opt out. I hope this can help you.
A support case was created for you and you should be contacted shortly by a Customer Support Representative, if you haven't been already. You can view and update your case at the Activision Support website. After logging in, go to "My Support" then "My Cases".