What do you mean KILL?
IF you mean the job is running and you are in a restricted state, you need to cancel the interactive session.
If you cannot do that, only other option is to POWER OFF. Not the best solution.
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Do you mean you are in a restricted state and the backup is happening, now you want to stop in midstream?
Depending on how your keyboard is mapped, there is a sequence (usually CNTRL ATL & Enter) that will interrupt your interactive job and give you and option to cancel the job.
You can always hit POWER OFF on the AS400 Control panel. Not the best option. :-)