To turn off email notifications for all users, change field pmnotify to 0.
For all users, this query in phpMyAdmin will suffice:
UPDATE `mybb_users` SET `pmnotify`=0
Or, make a note of uid for users who should leave notifications on (such as admin or moderator).
SELECT `uid`,`username`,`usergroup`,`additionalgroups`,`pmnotify` FROM `mybb_users` WHERE `pmnotify`=1 ORDER BY `usergroup`
In this select query, you can change pmnotify field to 0 manually, or note uid of exceptions, then update all as above, then browse the table and change individuals noted earlier back.
Then, to prevent users from setting notifications back on, edit the template set for each theme in use.
In AdminCP / Templates & Style / Templates , click on a template set to edit. Click on User Control Panel Templates to open up the list, then click on usercp_options_pms to edit this template.
find the following text and delete it.
<tr>
<td valign="top" width="1"><input type="checkbox" class="checkbox" name="pmnotify" id="pmnotify" value="1" {$pmnotifycheck} /></td>
<td><span class="smalltext"><label for="pmnotify">{$lang->pm_notify}</label></span></td>
</tr>
Do this for each template set in use for each theme.
These instructions are derived from 1.8.26 default theme. You may encounter differences if you're using a custom theme.