Did you get an email notification on this thread?
You may have had the automatic subscribe on, which would automatically subscribe you to any thread you posted in, but you didn't post in this one. Do you have a copy of the notification that I can see... this would be quite strange.
Anyway, it appears you have changed that setting in your User CP already... you musta just did that. This will stop any future subscriptions.
To unsubscribe from threads just go to
http://www.hometheatershack.com/forums/subscription.php and check all of the threads and select Delete Subscription.
Anytime you do subscribe, in case you do, you can always look at the very bottom of the email notification and you'll see something like:
Just click on that link and it will unsubscribe you.