I've solved the same issue with the script available at http://perishablepress.com/blackhole-bad-bots/. With this blackhole approach I collected a list of malicious ip, and then using .htaccess denied them. (Which is not mandatory, since the script itself does the banning. But I need to reduce the server load by avoiding php parsing for known unwanted ips) in three days my traffic came down from 5GB per day to 300MB, which is quiet expected.

I've solved the same issue with the script available at http://perishablepress.com/blackhole-bad-bots/. With this blackhole approach I collected a list of malicious ip, and then using .htaccess denied them. (Which is not mandatory, since the script itself does the banning. But I need to reduce the server load by avoiding php parsing for known unwanted ips) in three days my traffic came down from 5GB per day to 300MB, which is quiet expected.

Check this page also for full list of htaccess rules to block many known junk bots. http://www.askapache.com/htaccess/blocking-bad-bots-and-scrapers-with-htaccess.html