This module provides the Edit Blacklist ad-hoc command described in XEP-0133 and also performs the actual blocking of incoming and outgoing server-to-server connections.
In your client, simply select the Edit Blacklist command from the list of adhoc commands. E.g. in Pidgin, this is under Accounts -> (your account) in the menu.
With the plugin installer in Prosody 0.12 you can use:
sudo prosodyctl install --server=https://modules.prosody.im/rocks/ mod_adhoc_blacklist
For earlier versions see the documentation for installing 3rd party modules