Angry part:
1. Heard of a tool called "ping"? It is used for network connectivity test &
troubleshooting. With It anyone can bring your website down. Electricity that
you use is also used for killing someone in electric chair. So, you are using
the same thing that is used to kill someone. 
2. The best way to secure a site is to remove & disconnect all the wires from
the pc that is hosting the site. 
3. Did you know, most of the search engines have a copy of your website too?4.
When you live in a house, you ensure the security,  why wouldn't you make your
website secure before you host it?

Cooling Down: 
1. You should have consulted a security company. 
2. Even Xavier have been keeping a long FAQ on his website guiding how to
prevent what you are suffering from now. 
It is "Abuse FAQ for webmasters"
<http://www.httrack.com/html/abuse.html#WEBMASTERS>
These techniques  limits the capability of his own software. Still he gave
these techniques out just to help people like you. 
3. Think of it as an accident and it could have happened even if there was no
httrack. 
4. You can see the log if you have any to findout the ip that accessed your
site the most. Using that IP, you can take legal actions. 
5. Further more, the log is likely to contain the http headers. httrack writes
"Mozilla/4.5 (compatible; HTTrack 3.0x; Windows 98)" in the http header I
guess. So, http header containing that might give you clues.