Yes, there is a way to protect sites in general against grabbers like httrack.
if you detect a very high rate of requests  from one IP over a specific period
of time (maybe more than one request per second for more than 10 minutes),
then 
a) you can block this IP permanently
b) you can block this IP a specific amount of time (e.g. 24 hours)
c) ask the user to insert a code embedded in a graphic (captcha). 

so thats all
steve