If you are going to block all incoming and outgoing messages except for internal use only, as a System Administrator you can stop the SMTP service under Manage | Services. If you are only wanting to prevent incoming external messages ("Joke of the Day", etc.) and outgoing external messages for certain domains, then as a Domain Admin, edit the user(s) that you want to limit, go to the Service Access tab, and deselect "Enable incoming SMTP access" and "Enable outgoing SMTP access". The user(s) can still receive and send mail to local domains, but external senders will receive a 550 no such user and the user(s) will receive a bounce with 550 outgoing SMTP is not permitted.
You can also disable those two settings under Domain Settings | Advanced Settings | Default User Settings and then propagate those two settings to all domain users with the User Propagation option.
I hope this helps.