Found my solution.
In Outlook, go in the File tab. Under File, you have a menu.
At the right of this menu, you have a square button named Account Settings.
Click on it. À scroll box menu will appear. Click on Account Settings in that menu (first on top).
A window appears. In this window, you have a “RSS Feeds” tab.
Go in it, select the feed and click on the delete button.
I put some pictures. It should help even if it’s in french.