Go to 'User CP' on the top left hand side of your screen (in the blue bar underneath the 'Weight loss forum' banner)
On the left hand side, look under 'Settings and Options' and choose 'Edit options'
In the second column named 'Messaging and Notification', go to 'Default subscription mode'. There is a little drop down menu, and the first option is 'no subscription'. Click it and make sure to click 'Save changes' at the bottom of the page.