Once a print job has gone to a printer as far as I know you can't cancel it. You cancel print jobs from your computer software.
When you have pressed print you will notice a small icon like a printer on the bottom right of your screen. Double click on it and it will bring a print dialogue box up. Highlight the print job, click on FILE, click on PURGE PRINT JOB and that will cancel your print. If the printer still keeps going that is because when you order a print job the computer puts as much information over to the printer as it has memory and then the printer takes that info and processes it. So the printer will always have some info stored in it and you need to let that print before it recieves the command to stop.
Hope this helps.