The one you probably want is "Default Thread Subscription Mode", this is set to "Instant Email Notification" by default (I think).
With this setting turned on, when you post in a thread or start a new thread, you are automatically subscribed to the thread so that any new posts by other users will cause an email to be sent to you.
You could change it to only send emails once per day or once per week, or you can turn it off completely.
If you want to delete all your current subscriptions, you can "Empty Current Folder" to remove them.
Please don't mark emails from the forum as spam if they aren't - that can affect our ability to have genuine emails delivered to other members.
To unsubscribe from a single thread you are currently subscribed to, go to the "Thread Tools" menu at the top of the posts in that thread and select "Unsubscribe from this thread".