In the User CP page scroll down to the Subscribed Threads section on the left-hand side of the screen and click on List Subscriptions. This will take you to all the threads that you have subscribed to (every time you post on a new thread you subscribe to it). Once there you can check all the boxes on the right hand side of the thread names and go to the bottom of the list to the drop down menu Selected Threads:. There are many selections on the drop down menu that you can choose. Select one of the actions and then click the GO button next to it. If you have multiple pages of subscriptions you will have to do this on every page.
You can also change your preferences when you post by scrolling down the page to Additional Options. You will see this option:
You can choose from the drop down menu to not subscribe, recieve no emails or whatever you want to do.