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.
http://nsm08.casimages.com/img/2015/08/08//1508081247317901613493382.png


http://nsm08.casimages.com/img/2015/08/08//1508081247327901613493383.png