I have never used Knox, but I'm a little confused by your post. You say you installed knox from the Play Store, then uninstalled it, and now you want to get rid of it? Do you see the contradiction?
If you have a user-installed app you cannot uninstall, the usual thing is to check it has not been set as a device administrator (look in the system Security settings).
If you mean the knox component that came installed with the device, you certainly can't do anything without root, and I don't believe you can even with (certainly the "e-fuse" counter you won't be able to touch).