You should be able to Cancel a job that is "Ready" but hasn't started yet
The Duplicate phase of one of my regularly scheduled D2D2T jobs in Backup Eec 2012 failed becaude a tape wasn't available, so I simply right-clicked on it in the server's Jobs list and selected "Run Now" to try it again. It went to a status of "Ready; no idle devices are available" because another job is writing to the tape drive.
This is all well and good, but in this case I later realized the extra run wasn't necessary.
I wanted to cancel the extra run but the Cancel option was disabled. I could wait until the job becomes active and cancel it but the job is supposed to write to tape.
In order to keep unnecessary stuff from being written to the tape, I have to monitor the job and catch the magical window between when the job becomes active and when it actual starts writing stuff to tape.
It really would be better to Cancel the job as soon as you know you don't want it to run, before it becomes active.