User blog comment:Ssj3gogeta96/Ways to kill a soul reaper/@comment-5551986-20130601081059

Basically, Ichigo thought Urahara was simply a spiritually aware human, he didn't think Urahara possessed a spirit weapon like a Zapakuto and was actually a Shinigami. Only spirit weapons can harm shinigami, humans can't "really" even see then, except in a few cases. Ichigo couldn't cut Kenpachi because he solidified his spiritual pressure or "power" to make an "invisible armor". And when Ichigo first had a go, he didn't make his reiatsu sharp enough to cut him. So Kenpachi was just stronger at the time. When it says decapitation is the only way to kill a shinigami it means that they are durable, one sword slash might kill a human, but it could take hundreds to kill a strong enough shinigami like Kenpachi. As long as their body is intact they have a chance to come back due to their resilience. Shinigami are just like, "super-super humans." they can still die of illness and old age etc. Think of killing a human x10. More or less.