Yep do as JDsDream says, It was driving me nuts as well. This is what you do;
Go to, Control panel>
On the left side bar under "settings And Options" tab click on "Edit Options"
scroll down to "Messaging & Notification"
in that you will see "Default Thread Subscription Mode"
Click the drop down window and you will see "No Email Notification" select that and then go all the way down to the bottom of the page and SAVE CHANGES.