What's wrong with dynamically altering firewall rules? Before answering you should perhaps consider that firewalls can be used for tarpitting (i.e. slowing down connections to the point of unusability) or rate-limiting individual addresses or address ranges, as well as simple blocking. In fact, if you have to resort to an IP-based policy (generally a bad idea), a well-implemented firewall solution is usually a better idea than server-side request mangling.

To answer the OP's question, if you're on Linux you may want to look at the "recent" iptables extension. This article provides an introduction on how to use it. If you're on a different OS, have a look at that OS's firewall documentation.