1. Log into the Itunes store on your computer.
2. On the right under Quick links, open "Purchased"
3. In my case it was a TV show, so I clicked 'TV Shows' button on the top
4. I clicked the TV show I was trying to stop auto-continuing downloading and at the bottom of the screen unchecked the Auto download HD.
This will prompt a window that asks if you want to auto hide HD SD videos.
Hope this is applicable to your issue.
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