I have this same problem. In the past I have purchased t.v. shows that have both standard Def. and High Def. files included in one purchase.. I Only Want the S.D. version [for the 10th time] I don't care if I've already paid for it, for personal reasons [this should be enough] I do Not Want it!!! How do I permanenlty delete itunes incessant attempts to download these files [some 2.3 GB and larger] even after I have deleted them thousands of times, but they keep reappearing?!! This has frustrated me for Years!!!!! If I delete the saved S.D. file will this also delete the available downloads?
It turns out that this is not an uncommon problem. After a lot of searching on the net I came across these two solutions. In both you have to allow the entire song, video, etc. to download. Once the download is complete DO NOT sync your device, instead mark every one of the unwanted items as Read or Watched. Now sync your device (the files should not be copied to it) and go back to iTunes to see if the file is automatically deleted (as far as I know this only happens with podcasts). If the file is deleted your problem should be solved but if not select the file(s), hit Command + delete and choose the move to trash option for the file(s).
Restart iTunes and check to see if the same file is downloading again. If not, you should be okay but if it is then you're trying the second method.
Allow the file to download and do everything as in the first method except when it comes to deleting the file(s) navigate to your iTunes folder where the file is stored and physically delete it from there. Restart iTunes and see what happens.
I can't really speak for how well the second method works as the first method worked for me but I wanted to give both options in case it didn't work for someone else.