When users become Past users (ex. when a Parent becomes a Parent of Alumni), their notifications are not automatically disabled. 

To disable notifications (for users with access to their login):
  1. Log in.
  2. Click your name in the upper right corner.
  3. Select Settings.
  4. Click Notifications on the left. 
  5. Un-check any notifications you no longer want to receive and/or remove the Text Number by clicking the red X.
  6. Changes are automatically saved. 
User-added image

If the user no longer has access to their login, a Platform Manager can log in, impersonate the user, and make the above changes on that user's behalf.