2
Each user can control this by checking the box:
'Allow other users to view my email address'
under edit profile (or when registering).
This allows anyone to see their email address on the userinfo.php page for that user.
You could also edit the user table in the database by setting the field 'user_mailok' to 0 on any or all users. Although this would only affect current users and not new users in the future.
If you dont want users to be able to publish their email address on your site, you could edit the template for the userinfo page at: /modules/system/templates/system_userinfo.html and edit around line 58. Just change
<{$user_email}>
to:
OR
Not Available
Hope that helps,