In your User CP you can Edit Options to change your default subscription mode.
Once that is done you will have to change the already subscribed threads to not have an email sent by using List Subscriptions. Check the boxes (or use the box at the top to select all of them), scroll to the bottom and use the drop down to change the subscription type.
"First you start with a pound of bologna..."
-My Grandmother on how to make ham salad.