Also if you click on the forum link on the menu bar their is "View New Posts · Mark all forums read · Preferences".
What I did to disable auto-subscribing was to choose or click on Preferences for the forums, making sure the auto-subscribe was unchecked, then saved. Even if it is already unchecked, try saving again. If it comes back with a Preferences Updated message, you should not have to worry about getting auto-subscribed. It worked for me. Just note that you have to set the Preferences for each site's forums that you visit, as it does not appear to be a global setting.
