You right click the offenders IDS Blocks and block the /24 subnet.

This was suggested once, but this has not been implemented as of yet. You can however blacklist the CIDR block right from the IDS Blocks page now though as Brian pointed out :-) I wait for a number of IPs to show up in my block list and when I spot a common CIDR block I blacklist the whole CIDR and remove the rest of the IDS blocks. I hope that helps!

When I observed the abuse it was not triggering into the ip blocklist. I was in the smtp log for another reason and noticed the same /24 subnet kept attempting logins but only one or two attempts before moving to a new ip in the subnet. I hadn't seen that behavior before. 

For habitual and repetitive abusers, I use the Windows firewall to block IPs.  In theory, it should reduce load on the SmarterMail service.  I do this with iptables on my Solaris machines, as well, to take the load off tcpd, as well as full blocks in my border firewalls.

Just keep track of what you block, when you block it, and where you block it, as this will help save headaches later on when or if IP block ownership changes and you suddenly start blocking legitimate sources.