Hi. Thanks for taking the time to answer me. 

Hmm looking at your answer, I think I should precise that I give httrack an
url argument of the form
<forum_address>/forumdisplay.php?f=<numberofthesubforum> ..
followed by a filter chain of -* +*/showthread.php?t=*
That command basically does make httrack to catch only the threads belonging
to/displayed by that subforum (I checked the downloads) but at some point it
catches a few links contained in the body and signature of poster comments
that use the same expression as my url+filter and then escapes on these
foreign threads on the same forum, and one thing leading to another it ends up
catching threads everywhere.