How can I prevent the SD version of a TV show from downloading in iTunes when I purchase an HD show?
There is no way to prevent this permanently, automatically.

For each individual purpose, you can contact iTunes Music Store customer support, and as them to remove the SD version from your downloads queue, but bear in mind that you will not be able to transfer the movie to any iPods etc that only support SD (most of them, except for the very latest incarnations).

Source: Second result of a google search