I have added a slack service template to my gitlab-ce via the administrators interface. Everything worked. I activated "active by default". As a result, all projects now push notifications into the main channel. And those are a lot.
Changing the service template configuration is not inherit by the projects. Thus effectively rendering me unable to revert the setting via the admin UI.
So, how can I disable the slack service integrations for all projects before it drives all of us crazy because the general-channel is just flooded by gitlab?