Let's say that the hands behind his back are such a magical construct that Ner'zhul has no control over, and therefore if the Nathrezim want to chain him (or never give him freedom at all) they simply have complete access to control these hands (thanks to Kil'jaeden? ), and therefore let's say that the Nathrezim could chain their hands to the same place where the ice throne was, and the hands would simply hold Ner'zhul and he would not be able to free himself (+ pull the sword out of his soul otherwise he could probably destroy his soul).