If you own your server and have full access to it …
You can insert firewall rules to block access from the offending IP for a set time.

Inspired by a presentation on spam handling by merlyn I wrote up a program that monitors log files for unwanted activity and locks out an IP for five minutes when offending activity is detected. This alone cut the number of spams delivered to my system for processing from ~30,000 a day to less than 1,000.

It continues to be a great learning exercise that I'll hopefully polish into something real eventually.