As a work around, you can use the Custom Filter rules (under Settings > Custom filters) to quarantine messages from a specific Class C network. Your rule should search the Message header for the first 3 octets of the IP range (the first three sets of numbers).
For example, the following Custom Filter rule would quarantine messages from 93.184.216.1 through 93.184.216.255:
This will also block messages that passed through a server in the IP range.