From: DeskNow Knowledge Base

How can I blacklist some senders?

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 or from contains ... 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