Starting from your user profile page (which you get to by clicking on your name at the top of the screen), click on Subscriptions. You'll see a list of subscriptions, and you can unsubscribe from one by clicking its Unsubscribe button and then clicking Submit at the bottom of the screen.

 

If you want to stop receiving ChoralNet subscriptions altogether (or get it in a convenient index once per day) you can control that using the Email options menu near the bottom of the page.