When editing as a user there are really several areas they can set for emails.
One is preferences.
Send me an email when a new post to a forum, thread, or by a user within my Follow Lists is created
Yes No
From time to time we send out emails regarding this community. Do you wish to receive these emails
Yes No
From time to time we send out emails regarding this community. Do you wish to receive these emails
Yes No
They should be set to no.
Also below that section is a few extras depending on their status with the forum Like.
Moderator and Administrator Email Notifications
Email me when a post needs to be approved in a forum I moderate
Yes No
Email me when a user uses the "Report Post" option. These are usually sent only to Moderators
Yes No
Email me for user account related administrative actions such as:
- When a user accesses multiple accounts.
- Forum display name change requests.
Yes No
They should be set to no
Then in that users settings is follow lists.
They are follow user, posts, forum.
You would need to edit each tab and set all of the items to no notification or delete all of them.
But to be honest the preferences section should be all you need if you set all the settings noted above to no.
