The Blocked sender
feature allows IP addresses but does not
support IP ranges
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 18.104.22.168 through 22.214.171.124:
This will also block messages that passed through a server in the IP range.