Definitely use tire levers (otherwise you'll scratch and score your wheel), you may also want to try heating the tire somewhat to make it more plyable (hold it over a burner on your stove and be careful not to scorch it). If you plan on throwing it out, simply cut it off with tin snips or shears. I had to sacrifice 2 studded winter tires recently, cuting them with shears, as I simply could not get them off the wheel. Oh well.