In the "Log, index, cache" subsection of
<http://www.httrack.com/html/fcguide.html> it appears that adding
--cache=0
(that's a zero) to your "Scan Rules" may accomplish this task.

I'd be curious if the command line generated (in the log file) when you
uncheck "Use a cache for updates" contains the "C0" option.  Because, if so,
putting the option in the Scan Rules is unlikely to help.  There may be some
other issue going on.

If you have to post back, try including the first few lines from the log in
their entirety.  That would help diagnose.  You may even spot it yourself if
you do that LOL

Also note that there may not be much benefit to doing this unless disk space
is really a problem.  I regularly remove the cache by deleting everything
under hts-cache except winprofile.ini and have not had any trouble.  Not sure
if your GUI is "Web" or "Win" but even if you had hundreds of mirrors,
searching and removing new.* and old.* shouldn't be much of a hassle. 
Regardless, if you have your reasons... ;^)