Greetings @JeanMJ and welcome aboard 
Currently, the only workaround is to put “Cancelled” next to the widget title:
However, if it was possible to add a banner with the word “Cancelled,” it would look much cleaner. We have a request for this feature on the Wishlist, so feel free to vote for it to stay updated - Banners for events
Let me know if you have any questions left 