Go to your “My Groups” page
If you see that group listed, click on “Edit My Groups”. Locate that group and in the “Message Delivery” column, use the drop down arrow and select “No Mail”. Then, in the last column titled Leave Group, click the square box to put a checkmark in it. Click “Save Changes”. This will stop the messages from coming to you and will remove you from the group.

Click “edit memebership” at the top of the screen when you are in the group. Then after that screen appears, go the the bottom right and click, “leave group”. Make sure you also click confirm or it will belike you did nothing.