captcha and blacklist

Hi, I'd like to set a captcha on my web server(s). The code works quite fine, but I'd like to blacklist (for an amount of time) the ips that failed with the captcha x times. The web server works with an SQL server, but I cannot ask the page to check for each IP with a request to the database. I can't also store the blocked IP to a filename on the server, because the site is served by several http servers. I thought about a DNS server for the blacklist (the same way it works for antispam), but perhaps someone has a better idea ? thanks N F