I'm having the same problem...most sign-ups through the newsletter form in my Minimal footer are not genuine. I can either sort them manually or risk my Mailchimp account by testing each email with a welcome letter. They aren't getting sent over to Mailchimp anyway, and I can't always figure out which are real, and my real customers are not getting a timely email. It's pretty terrible. Worse than not having a sign up. How could this continue?
Surprised no one's responded with an answer here yet. Here's how you activate/deactivate double opt-in with MailChimp:
Hope this helps!