Before deleting the notification, make sure you've disabled that integration from the notification's data-source section
Step 1: Go to Notifications, edit the notification & go to data-source & disable it.

Step 2: Go to your 3rd part app or tool where you've added the webhook & delete that webhook or connection.
Step 3: Go https://app.wisernotify.com/integration , find the integration & delete the webhook or connection from wisernotify.