Assuming the software is Apache, you can put a .htaccess file in your main directory with the following:

ErrorDocument 403 /banned.html
order allow,deny
deny from 123.123.123.
allow from all
[download]

Where 123.123.123. is whatever part of the IP you want banned. You can put in as many deny lines as you want, though excessive use of deny will likely slow the server down. The best thing to do is run a script every so often that checks your server log for x number of requests within x number of seconds during the past 30 days or so and edits those IPs into a ban list that you can cut and paste or edit into the .htaccess using a second script (depending on whether you're running the first script on or off the server).