The above cancellation only works on Email Programs. If you are wanting to cancel a smart campaign follow these instructions:
Note: A smart campaign will only stop what hasn't been run yet. If you already had 100 emails sent by the time you cancel it, those 100 emails cannot be unsent.
You may see in your Outlook an option for "Recall Email." This function within Outlook only works with internal sends/private systems where the client can request the server revoke access to other client mailboxes before they're viewed, such as your office email to other users in the same company.