If you want to block specific hosts yourself rather than use a blacklist (in other words, create your own blacklist), you can configure a class of service that prevents messages from those hosts. You do this on the Internet Agent object's Access Control Settings page by editing the desired class of service to add the hosts to the Prevent Messages From exception list on the SMTP Incoming tab. For example, if you wanted to block all messages from badhost.com, you could edit the default class of service to add badhost.com to the list of prevented hosts.
For information about editing or creating a class of service, see Creating a Class of Service.