At the moment there's no way to do it officially however KuJoe on the InterWorx forums has done a cool script you can run with the cron jobs: http://forums.interworx.com/threads/7931-Stop-Outbound-SMTP-Automatically-on-Mail-Queue-Size-Alert?p=25995#post25995
Or you can use spamdyke.org We will be looking more into it and will do a tutorial on how to install it however you can use SpamDyke at your own risk.
