Hello Marta,
From my experience, I have never seen a way that you can delete only a particular job from the Completed Job List. Even the CUPS web interface (
http://localhost:631/printers) does not provide a method for deleting a completed job.
From reading the link to Kappy's reply, it appears that he was referring to when the job still appeared in the default printer queue view, not when you select Show Completed Jobs. However I have never experienced a job that is completed still displaying in this view, so this may have been a particular user issue.
Unless someone else does know of a method to delete an individual job, I believe the only option you have is to delete the entire list by resetting the printing system.
Pahu