If you're talking about items in the print queue, they normally automatically delete once they are printed. If they are not being removed from the queue either, either they haven't been printed yet or something is wrong with your system. You could try opening Printer Setup Utility (or Print Center) and choose "Reset Printing System". This will remove all of your printers, so you'll have to re-add them. It's best to reboot after the reset and BEFORE adding your printers again.
Another option is to see if there's a new driver available for your printer / OS from HP.