Go to edit options in your usercp and make sure that the drop down menu near this paragraph:
Default Thread Subscription Mode When you post a new thread, or reply to a topic, you can choose to automatically add that thread to your list of subscribed threads, with the option to receive email notification of new replies to that thread.
Default Thread Subscription Mode

is put as "Do not subscribe". To get rid of the ones you are subscribed to now just follow the link in that paragraph and remove them there.