NEW EDITION OF MASSMAILER
Navigate to the respective Campaign Details Page and Click the 'Unschedule Job' button. This will unschedule the campaign provided it is still not completed yet.
Here is the article that explains how to do the same if you sent the email by other means in MassMailer.
OLD EDITION OF MASSMAILER
Navigate to the respective Campaign Details Page and Click 'Unschedule MassMailer Email' button. This will unschedule the campaign provided it is still not executed yet.
