Yeah I've wondered about this one before, I know it's been discussed but I didn't turn anything up.
I agree with the above sentiments that, from the text, we can really only assume it reached a sort of stalemate before Zioz was "damaged" enough that he was unsummoned/banished.
H., I also cannot remember any instance where we've ever actually seen one killed...if that's even possible? Akka did manage to beat Ankaryotis into submission when escaping the Scarlet Spires...
Lame as this explanation may appear, it does basically seem that you simply have to kick their ass to a certain point where they default to the Outside.
I seem to recall at least one Ciphrang being hit with a Chorae, but again, I think it just banished the demon from the World (no salting or anything). I could be wrong about that though, it's a bit fuzzy.
Hell, even the Ciphrang heads hanging from Kellhus's belt still seem "alive" (I'm honestly not sure terms like life/death really even apply to a Ciphrang, or any other entity native to the Outside).
ETA: On the concept of "beating a Ciphrang into submission": perhaps the key to this is simply a battle of wills? The Ciphrang, for all their might, are always described as utilizing either raw strength, or otherwise a form of sorcery that seems comparable to the Anagogic variety. Wolves, flies, rats, suns...their "magic" is always represented by something Worldly, never in the abstract sense of the Gnosis. There's no way this is a coincidence in my opinion, though I cannot conceive any good explanation for why this is the case. Regardless, I suspect a battle between a mortal sorcerer and a Ciphrang ultimately comes down to which one of them can overpower their foe's perspective, or at least their enemy's frame of reference. For example, when Akka "defeats" Ankaryotis, perhaps it's not because he really caused the creature any meaningful, material damage -- but instead that Akka's power and focus at that moment was enough for him to establish his own "frame"...his mastery of the Gnosis is such that he can theoretically overcome anything the Ciphrang could possibly throw at him, since the latter seem to be inherently (for whatever reason) restricted to a form of Anagogis.