think there is no way around this problem, u can only soften the effect by lowering your flash power or using bounce flash. how u bounce up to your creativity, wall or bounce card or ceiling or umbrella.
LL if u only got 1 flashgun - a basics about light is that the larger the light emitting surface the softer the shadow it cast, your 580ex2 the head surface area compare to a human is super small so if u can increase the surface to much much bigger to cover maybe half the human body u should get very nice soft shadow.
else the other way is to use multi flashgun, 1 to light up your subject and the other to wash out the shadow. but this will take alot of trial and error to do.
;p hope your subject was ok after you threw your 580ex2 at him or her, but i hope you missed and no one was hurt maybe less the 580ex2. :bsmilie:
how can it be NO shadow? unless you subject is transparent.
harsh shadow come from harsh lighting, increase the size of light source relatively to your subject will yield softer shadow, hence bounce light toward ceiling is most people will do. when there is no ceiling to bounce, get a large softbox is way to go.