Go to the User CP (up at the top of the forum - in the web version) and open/click on it. Then, go to the line Edit Options, which is under the heading Settings & Options (these are in the left column).
Scroll through the selection available (on the right hand side) until you find/come to a box titled "Messaging & Notification." In that box is a line titled Default Thread Subscription Notice. There is a drop-down box in there - select which option you wish (one of which is "No Email Notification").
That should do it. Good luck.
Marilyn