After execution, the JOB Command sets system variable $ZCHILD
$ZCHILD contains the ID of the last child process that the current process created with the JOB command. If your process has not used JOB to create a child process, $ZCHILD returns 0 (zero).
$ZCHILD being set does not mean that the job was successfully started. It only means that the process was created and the parameters were passed successfully.
Now you have the OIID in hands to apply to $System.Process.Terminate(bgjob) as suggested already by Vitaliy Serdtsev