I'm trying to figure out how to stop IPB from sending out emails to users who are following specific forums. Unfortunately out of the few people that actually use the function, most of them are either not active or their emails don't exist anymore so they're just bouncing.

Any way to completely deactivate this feature?

1.  You can adjust the notification preferences for new users in the ACP.  

 

2.  This will not adjust the preferences of existing members.  You will need to unfollow existing followed content manually or run an SQL query.  

3.  There's a Mail Bouncer app available in the Marketplace that might benefit you as well.