If you want to unsubscribe from a certain thread, just open that thread and click 'Thread Tools' which is above the first post in a thread, then click 'Unsubscribe from this Thread'.
You can actually also unsubscribe from threads by clicking 'Subscribed' which is under your username at the top left and then tick which ones you want to be removed and then under 'Selected Threads' choose 'Delete subscription'.