You can set the default notifications sent from SocialEngine PHP by following this guide. This can help control which emails are sent from your site for new users. Note that this only controls the email notifications on initial sign up. Individual members can change these settings in their Email privacy section.
You'll need to be logged into your Admin Panel.
Navigate to Settings > Default Email Alerts.
Check to enable or uncheck to disable the email notifications and Save Changes when done.
Remember to clear your cache!
This will only work for new members and will not affect current members.