If you can afford using mod_perl, a commonly suggested solution over m_p mailing list is a module capable of blocking greedy clients. A great example of such module can be found inside the famous Eagle Book, and (lucky you ;-) ) also in a sample chapter online here. A little summary:

A trick that some mod_perl developers have used to catch devious robots is to block access to things that act like robots by requesting URLs at a rate faster than even the twitchiest of humans can click a mouse. The strategy is to record the time of the initial access by the remote agent, and to count the number of requests it makes over a period of time. If it exceeds the speed limit, it gets locked out. Apache::SpeedLimit (listing 6.4) shows one way to write such a module.

I think there are many modules listed on CPAN that can do something similar.

Ciao, Valerio