You can't kill a Devil Fruit user with that thing, Yes It hurts, but it won't kill them, + You have to Haki to even touch a Logia type, The degree of "weakness" induced by the "sea stones" depends on its density and composition; for example, "Sea stones" handcuffs rob the prisoner of their abilities, but still allow the prisoner to walk around normally. It only works upon direct and constant contact, which explains how Smoker can carry a Kairoseki-tipped jutte on his back, without it disabling him (presumably, thick clothing will prevent the Kairoseki's effect from harming the victim).