It will be tricky to say how to specifically turn of notifications without knowing exactly what kind of emails are being sent. Are your users using mailing list mode? Or do they have tracking set up for some really active categories? I’ve seen these 2 reasons to be the case for large emails being sent.
To turn it off completely, you could set the default email level to never, but that would mean users would need to log in to the site to know what’s happening. You could also disable mailing list mode on your Discourse site and a few other settings instead, I’ll suggest going through the settings listed in the User Preferences section of your site’s admin settings /admin/site_settings/category/user_preferences to get a better sense of what you can adjust.
Also, go through this post for an overview of these emails and the options you have with them.