Where did you activate this option from?
In Setting of plugin bbPress Notify (No-Spam) Settings (v2.15.2)
On Topics tab:
bbPress Forums Subscriptions Override Override Subscriptions to Forums.
Enable this option if you want bbPress Notify (No-Spam) to handle bbPress subscriptions to Forums (new topics). The bbPress Setting “Allow users to subscribe to forums and topics” must also be enabled for this to work.
Click here to learn more.
Automatically subscribe new users to all forums.
Enabling this option will make it so that users get subscribed to all forums the moment that they’re registered They can unsubscribe from the forums later to stop receiving topic notifications.
Automatically subscribe all forum subscribers to newly created topics.
Enabling this option will pull all of the forum subscribers and automatically subscribe them to the new topic so that they get notifications of new replies as well. They can unsubscribe from the topic later to stop receiving reply notifications.