We know from the Luccio in the Wild West short story that the Wardens had chopping Kemmler as a high priority objective as of ca. 1870-ish. They actually did kill him repeatedly, the problem was he just kept coming back, presumably through the same or similar means as Team Harry interrupted Corpsetaker from managing. What took them time wasn't hardening their attitude, it was figuring out a method to keep him down for good.
It isn't clear though how they finally did, or I don't remember it. What I remember is that a bunch of Wardens together took him down, but I don't believe it said just how they took him down. Actually I don't think they did, as I said a couple of posts ago, I think Evil Bob is actually Kemmler.
My tin hat theory!What if Bob has been Kemmler all along? Bob is a spirit, can he be killed? Don't think so unless maybe Mab can do it. Is that why Bob fears Mab? Or is it because Mab knows who and what he is? Anyway, so the Wardens "kill" Kemmler, but they don't really because he retreats in his spirit form, or Bob. Bob is very useful so a Warden like Justin steals him for his own use. Justin was a Warden, he wasn't always evil, but what if having "Bob" in his possession corrupted him? Turning Justin warlock? "Bob" would know about star born, "Bob" would have plans for such children.. Monkey wrench into the plan, Harry Dresden. Harry kills Justin, takes the skull and names the spirit, "Bob." Just naming the spirit begins to transform him, also Harry orders him to forget the evil stuff he knew before Justin died. Thus it was until Harry needed information about Kemmler and ordered a reluctant Bob to call him up. Evil Bob emerged and nearly killed Harry, after that Bob was so horrified he cut himself off from his evil "memories" for good. Those memories became Evil Bob who we met in Ghost Story, or was it really Kemmler?