there's an option in your User CP that enables that feature. to turn it off, go into your User CP and then select 'Edit Options' from the menu on the left. under the 'Default Thread Subscription Mode', you find a menu that controls the settings for your thread subscriptions. this is likely set for 'Instant Email Notification'. change this to either 'Do not subscribe' or 'No email notification' to fix the problem.
See the struggle of the faithless lot as they negate their time
How low to sink to the depths of their frame of mind