Incoming Webhook Webpart - How to manage the cards

I added an Incoming Webhook webpart to a section on a SharePoint site page.  It gives me a URL to provide to the service that I want to push notifications from.  

It works great.  My only question:   How to I manage the cards that get created in the webpart.  I'd like to delete test cards that were created during testing.  Where do these cards reside within my SharePoint site?  I can't find any new lists in contents.