Killing sendmail will effect other applications and your server.
A better solution would be to disable the mail function (Assuming your sending emails using PHPs mail function) only for your application.
To disable the mail function just for your application you can do this.
ini_set('disable_functions','mail'); //Place this in your PHP script
php_value disable_functions mail //Or place this in your .htaccess file
To disable the mail function for all your applications you can place the following line in your php.ini.
disable_functions = "mail"
Also, note that doing this might throw an error or warning depending on your coding.
Hope this helps.