Forum:KB:Darth Nihilus' death

If Nihilus is basically a spirit posessing sith robes and a mask, how could he be killed? Spirits can't be killed can they? I mean, how could the two jedi kill him if he's already dead? Is it that Nihilus needs the robes and mask to maintain a form and if he's defeated, his spirit must go with the force? I don't really understand, could someone explain how he died if he's a spirit?