To create a domain-level blacklist

Simply login as admin and click on Administration/Content filtering/Incoming mail

To create a user-level blacklist

In your account, simply click on Mail/Filters and create a filter like 'If from contains joe@domain1.com or from contains @domain2.com ... then delete the message'. You can add as many addresses as youwant.

To blacklist sending IP addresses

Login as admin and click on Administration/Mail/SMTP Incoming/Traffic control