Disable Outbound email or change SMTP server with an SQL Query?
In civicrm.settings.php, add the following:

define('CIVICRM_MAIL_LOG', 1);


This will route all email to files/civicrm/ConfigAndLog/mail (Drupal) or media/civicrm/ConfigAndLog (Joomla).

You can also specify:

define('CIVICRM_MAIL_LOG', "your_file_path");


to set a specific output file.

See https://wiki.civicrm.org/confluence/display/CRMDOC/Debugging+for+developers.

Another option is to perform the following queries to kill any scheduled or queued mail:

UPDATE `civicrm_mailing_job` SET `status`= 'Complete' WHERE `status` IN ('Scheduled', 'Running', 'Paused')
DELETE FROM `civicrm_mailing_spool` WHERE 1