I turned off content notifications for all users for Notify some time ago because it was sending duplicates. There was a checkbox somewhere that I unchecked. It is not in notification settings.
This is Drupal 6.
Reading your comment on my question above, I guess you're using Actions & Triggers in Drupal 6.
To see which actions you have defined or to define new actions, go to Administer > Site Building > Actions. There e.g. you can define an action for sending an email. From your question I think you already have done that (in the past).
To assign this action to an event, you have to go to Administer > Site Building > Triggers. There you can choose (under Content) to assign an action to an event, such as published or updated content.
There is no master checkbox or switch for the admin in Notify (there is one for each user).
To turn of notifications for all users with one click, you disable the module, to turn it on again, you re-enable the module. This is the same for the Drupal 6 and Drupal 7 versions.
As for the per user master switch, is can be found under My account ➔ Notification settings, as shown on the screenshot below.
For more about the use of this module, please see the community documentation.
The documentation I was looking for is here: https://drupal.org/documentation/modules/notify
The "master switch" appears to be a feature of the D7 version of the module.