USER CP > EDIT OPTIONS (in left hand column). About 1/3 down the page change default thread subscription mode to "no email notification".
This will not stop emails for new replies to threads you are already subscribed to however.
To change that, go to:
USER CP > LIST SUBSCRIPTIONS
You will need to check off all your subscribed threads (you can select all threads on each page by checking the radio box at the top) and then towards the bottom, where it says selected threads: change the option to "no email notification". Click "GO".