DDoS protection module suggestion

[malte]

Yes, I am aware that you can limit the max connections with the limit
zone module.
What I'd like to do is establish an in-memory list of abusive IPs, which
gets no responses other than 503 errors no matter what request they
make. Since they are abusive, I want nginx to automatically spend the
least amount of processing power on them as possible.

Posted at Nginx Forum: http://forum.nginx.org/read.php?2,147105,147117#msg-147117