To prevent users from turning off or deleting a subscription to a notification, make
the notification mandatory.
Open the appropriate notification.
Configure the form and add the Mandatory field. This
field does not display by default.
Select the Mandatory check box.
That notification is now locked in user preferences, preventing the user from
removing or unsubscribing to the notification, filtering it, or changing the
schedule. In the user's notification preferences, the control button for the
mandatory notification is on and is read only.