Customize keymaps - comment | uncomment toggle

Well, I am getting a bit frustrated. I tried to unbind the key for commenting in edit mode following the instructions that are given here: http://jupyter-notebook.readthedocs.io/en/stable/extending/keymaps.html - but the unbinding never took place.

Consequently, also the re-binding, for instance to "Ctrl-7" never worked as well (German keyboard).
So could maybe someone please provide an example configuration or tell me whether this is unexpected behavior or a case of "uts" (user to stupid)?

I ve been playing around with this for a while now and cd not solve it.