Okay folks, there is absolutely only one way, and one way only to make iTunes stop downloading things, filling up your hard drive, and crashing your system. EVERYTHING ELSE DOES NOT WORK.
You basically need to make it impossible for iTunes to write to its download area on your hard drive. This is a drastic solution, but again, EVERYTHING ELSE DOES NOT WORK. iTunes in its current state is a completely failed, broken product.
Note: this will delete all downloaded iTunes movies/tv shows currently on your Mac. This is the trade-off I had to make until Apple fixes iTunes.
1. Go to the command line.
2. cd Music/iTunes
3. sudo su (enter your login password)
4. chown root:admin iTunes\ Media
5. chmod 755 iTunes\ Media
5. rm -Rf iTunes\ Media\Downloads
6. rm -Rf iTunes\ Media\TV\ Shows
7. rm -Rf iTunes\ Media\Movies
Now when iTunes starts, it will try to download all of your content (even though you've told it not to in Preferences it does it anyhow). However this time it will display an error: it won't download the content, it won't fill up your hard drive, it won't crash your system when your hard drive fills up, it won't clog your Internet connection for 8 hours--it will just die, like software written this way deserves to die.
And you can use the rest of iTunes as you do normally.