In reference to this locked thread: https://www-secure.symantec.com/connect/forums/how-cancel-backup-job-if-status-readyno-idle-devices-are-available#comment-9228761
I wanted to say that there is a little workaround way to cancel a job that is in the state "Ready; No Idle Devices are Available" without having to put it on hold. If you right click the job -> "Edit" and then under Backup click "Edit", then click "Ok" and "Ok" (notice you don't have to actually edit anything), the job will reload and cancel the pending backup, going back to the "Scheduled" state if you have it on a schedule. Please note this action will also reload any other scheduled backups under the same job, so if you have Daily/Weekly/Monthlies all in the state "Ready; No Idle Devices are Available", they will all be reloaded to "Scheduled".
I needed this workaround because I didn't want the job to remain on hold because I needed it to run as scheduled, and I figure maybe it will be helpful to others.