Click on User Control Panel (upper left). The on the left of the default view, there's "Manage subscriptions". To avoid future notifications, you can change the default for being notified under "Board preferences" -> "Edit posting defaults". Also, with each post, you can check/uncheck the box to be notified for replies. And at the bottom left of EVERY topic, there's an "Unsubscribe" option. Also, if you ignore the emails, others are not sent again unless you visit/view that topic again.