If you don't have it already ensure your site is logging IPs. You can do a free IP WHOIS at www.dnstuff.com to see where IANA thinks the IP Address originates. In many cases it also provides the registrar or ISP for the IP Address and you can contact them directly to report it.
Obviously you can temporarily block the IP Address, the only problem with that is so many ISPs utilize DHCP addresses that even though the attacker has that IP today it could be different tomorrow and more importantly a legit user may get the blocked IP.
Where is your site hosted? If the attack took place within a period of time, say 10 minutes it should have triggered a DDOS alarm somewhere since the normal volume of the site is probably not that many requests in that short of a time period. Devices like Barracuda makes are designed to essentially block those requests when they come in too fast. IIS also have a similar feature where if too many requests arrive at the same time it will think it is being attacked, and in many cases will dump the connections. Many SharePoint search installations have this problem because the search indexer reqeuests a lot of stuff very quickly.
Hopefully this helps a bit or gives you some ideas as to what to look at. You can add CAPTCHA and other stuff to the site, but in the end attacks like this come down to TCP/IP and devices to recognize an attack and prevent or kill it, your website can only do so much to protect itself.