For jobs running on a tab, you can click the cancel button (red square) to cancel the job. You can also cancel a job from the job history window. Note that some jobs cannot be cancelled. The following job types cannot be cancelled:
Run Action Script
Blend to Saved Version (if the blend target is “Copy To New Version”, then the job can be cancelled, otherwise the job cannot be cancelled.
Domain Assign Membership
Put Nodes
Version Delete
Version Save