The status display in Your Broadcast History shows a summary of whether the job is scheduled, in progress or has been completed. It also lists the Job Name, list used, number of destinations (items), when the job was received by the system, when the attempts were started and finished. The last column shows "Cancel Job" if the broadcast is still pending a scheduled delivery or "Resend" if the job has completed all attempts. Clicking the "Submit Cancel Request" button will submit the cancel request to the system for the job in question.
If the deliveries of the message have not yet started (such as in a scheduled job where the start time has not been reach) then it is possible to cancel the job in question and no deliveries will be attempted. However, if the job has commenced deliveries, you cannot cancel the job; it will run to completion.