It sounds like you may also have Roles set to notify your users. That’ll force notifications to all users of the groups that have access to the forums.
You can try de-selecting the Roles and just enabling the Override Subscriptions options, or you can leave the Roles on and get the Opt-Out add-on, which will let your users choose whether to only receive notifications that they’re subscribed to, or to block all notifications.
Cheers,
Vinny
