If you find that you are receiving email notifications whenever someone from your group posts a message in the discussion forum, there are three different ways you can unsubscribe.
Each email notification you receive will contain a link to unsubscribe from email notifications - either from one specific forum or all forums on the site. You will need to enter your login details after clicking the relevant link.
You can login to this site at anytime, and click on the 'Edit profile' link in the 'Welcome' box on the right hand side of the screen. Go to the drop down menu called 'Forum auto-subscribe' and select 'No, don't automatically subscribe me to forums'. Then, scroll down and click the 'Update profile' button.
If you do not wish to switch notifications off altogether, you can switch them off for particular forums only. Each time you post to a forum, you will see a drop down menu called 'Subscription'. Select the option 'I don't want copies of posts to this forum'.