These couple of steps quite effectively kill an API Umbrella instance (at least the one I am using on Azure):
- Create a new API Backend
- Use the host name of the API Umbrella server as frontend ("listen to") IP, e.g. api-umbrella.myhoster.com
- Add a pattern for the API
- Publish the API
This immediately kills the Admin and Portal pages; all pages are rendered as generic 404 not found pages. The actual API will work though (I checked that using a REST client).