Good point, I wasn’t actually aware that we could block entire ranges, so great to know! Thanks Jeff! 
We are certainly banning each spammer’s IP address, but they use a different one every time - banning ranges makes a lot more sense. Initially, we wanted to be as permissive as possible, without leaving out possible legitimate users from those ISPs - but that’s less and less of a concern.
For those wondering how this is done, you can block ranges of IPs like this:
39.36.108.135
39.36.192.11
39.36.195.54
39.36.195.54
39.36.214.120
39.36.217.148
39.36.217.154
39.36.227.212
By entering “39.36.0.0/16” here:
This tool is useful for converting a start/end IP range into a CIDR which Discourse expects: http://www.ipaddressguide.com/cidr#range