For some time, I have had problems canceling a print command on my HP 722C.

Normally, I would double click the printer icon and select "Purge print documents." This wouldn't work.

I finally found the answer on the HP website. They say:

Open the printer's cover while it is printing. A message will appear on the screen stating Printer top cover is open. Select Cancel on this message to cancel the print job. Close the printer's cover.