Here's something I picked up in my /. travels, posted by vandan today. I modified it a little, but it still does basically the same thing. The blacklist is a text file in which each line is an IP address. I keep it in /etc/blacklist. Very simple, very small, and in the words of the original author, it works :)

(BEGIN SCRIPT)
#!/bin/bash

# variables
BLACKLIST=/etc/blacklist
IPTABLES=/sbin/iptables