How can I prevent iTunes from automatically downloading podcasts?
For anyone coming across this now, in the latest version of iTunes (~v12.5.1) it's much easier to prevent downloads for all podcasts you're subscribed to. 

Steps:


Go to "Podcasts" in the menubar dropdown. 
In the second column of your subscribed podcasts, at the bottom there's two buttons, "Refresh" and "Defaults...", click "Defaults..."
In the popup, set "Download Episodes" to "Off".