Banning trackback spammers

So recently I started moving IP addresses found in Movable Type's mt_tbping table to mt_ipbanlist, with the view to turning the trackback spammers invisible. As I understand it, IP addresses found in mt_ipbanlist are sent away with a 403 error or otherwise do not show up in the traffic log. I found a list of banned IP addresses online and added them to my mt_ipbanlist table. Now I have over 26 000 banned IP addresses and yet I still get about 5 spammy trackbacks per day. How many spammers are out there?

Banned IP list