If you don't know what qmail-spp is, please check it out. It makes qmail much much better.
If you know qmail-spp, then maybe my plugin collection will be handy for you.
My most useful plugin is probably ipthrottle, which you can use to make overeager IPs connect less often.
The version currently in SVN will autoblock those IPs for a configurable amount of time if you are using ipsvd which is like tcpserver, only much better.
I really need someone to help me test the SVN version, which should be way, way better than the releases on the page.
The SVN repo is at googlecode