Does your raw access log display a host name of “0”, or zero? Very odd, is it not? I have been struggling with this for a couple of months, and my ISP Site5 had no answers. It turns out that one of my spammers, NFORCE_ENTERTAINMENT, puts an unprintable character into their host table, so that when my ISP looks them up, they display the unprintable character in my log as “0”.
Trying to control your site’s spam can be challenging. If you try to ban an IP that is simply 0, or a host name of “0” you will fail, because there is no zero in their host name, but an unprintable character. Ban these guys instead.