It's not an elegant solution, but go to the iTunes store, click on your ID in the upper right hand corner, then in the dialog box, click Sign Out. Then nothing will download, but you also won't be able to purchase and download anything of your own. If that's not a problem, then it's a better solution.
For TV series and Movies, they should only ever download to your computer if they are 'Unwatched' episodes/features. If so, go to the TV Shows and/or Movies section in iTunes (under Library pane on the left of your screen). If there are 'Unwatched' episodes, a little red circle with a number in it should appearin the corner of every TV show cover. Just right click each cover and select 'Mark as watched'. They should stop downloading then.
As for music, I suppose Jon_E may be right. And it's not elegant, but that's iTunes! Why Apple outsourced the development of some of it's most heavily used software to a third party developer is beyond me. It should really be a little more intuitive. Surely they've received plenty of feedback to that effect?
If you sign into the itunes store and then view your account by clicking on your username in the top right hand corner. Enter your password when prompted.
In the second box down titled iTunes in the cloud it should say you have x amount of devices associated with this id. Click view devices.
from here you can just remove the device you want from auto downloads.
Hope this helps.
Thank you Phil! I've been having this issue, too. I've been trying to stop auto downloads because I'm using a public wifi with limited bandwidth and have a smaller HDD so saving space is important for me. I just wanted to use my Windows PC as a cloud-based iTunes device while still synchronizing will all my podcast subscriptions, etc. Thanks again! It finally worked.