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Display any of the spooled joblogs, and look at the headings on the first page. The job description name and library will be shown near the top left. When you know the job description to change, run the CHGJOBD command to set the future spooled joblog to *NOLIST:
CHGJOBD JOBD( jobdlib/jobdname ) LOG(*n *n *NOLIST)
It won't affect the current instances, but new instances should only spool joblogs if significant problems show up.
The joblogs might have been started in the past in order to watch for a problem, and they were never turned back off.