Kazu can do absolutely nothing about Susanoo. We already know that those variants, the deities, just go back to their home dimension if the possessed body is destroyed. Kazu might thus "kill" Susanoo, but the dude would just come back later, as if nothing had happened. In the meantime, Susanoo can kill humans for good.
I guess the best thing Kazu could do, before figuring out how to destroy the deities permanently, would be to go around Japan and erase every single Susanoo shrine. That ought to piss off the deity. And he could do absolutely nothing to stop Kazu, just like Kazu can do nothing if Susanoo wants to erase Yui or his little sister.