This is the setting that I am talking about. I have unchecked the "and receive email notification" box on every thread.
I am not sure where you were when you captured the image that you provided. I would hope that unchecking the box for email notification would turn them off. It does not work that way, at least for me.
EDIT: The problem is that my browser does not display the second radio button .
Reading your edit, does that mean that you were able to stop the email alerts for that thread by using a different browser? Which browser were you using? I'm currently using Chrome on OS X. These photos were taken using a basic test account in Firefox, still on OS X.
I got to this specific page using the "Subscribe/Unsubscribe" link found at the top of every thread. I'm wondering if this option and the one you showed in your screenshot are somehow different.