To unsubscribe please visit https://lists.wikimedia.org/postorius/lists/cloud.lists.wikimedia.org/ , and under the "Subscription / Unsubscription" heading there's a "Log In" button. Don't worry if you've forgotten your password, there's a "Forgot Password" option on the "Log In" screen.

Once you're logged in, there's a button labeled with "Unsubscribe".

Hope this helps.

-Adam