How can I stop Drupal from sending e-mails of any kind?
As another, quicker, option, you can add the following lines to your site's settings.php file (if you have the Devel module installed, which you certainly should if this is a dev site).

$conf['mail_system'] = array(
  'default-system' =&gt; 'DevelMailLog',
);


That will replace the default mail system with Devel's development mail system, which writes emails to a file log rather than sending them to a recipient. By default the files are stored in temporary://devel-mails, but you can alter that by setting another variable, eg.

$conf['devel_debug_mail_directory'] = '/path/to/folder';