Select the list where the scheduled message is located by clicking the Manage button from your list on the dashboard.
Click the Messaging icon in the list navigation bar in the top right hand corner of your list homescreen.
Below the green New Message button, you'll see a link that says Scheduled, click on that link. This is where all your scheduled messages that are either after opt-in messages that are sent to new subscribers, or scheduled messages that have yet to be sent to all subscribers will be stored. These instructions are just for stopping a scheduled message to all your subscribers. If you're looking to stop a scheduled message that is being sent only to new subscribers, check out this post.
To stop a specific scheduled message, click the Cancel link corresponding to the specific message you wish to edit.
When you click Cancel, they'll be a pop-up that appears to confirm you really want to cancel that scheduled message. Click Ok. That's it!