1. Go to My Accounts tab
2. If you have several accounts, choose the right one in the dropdown list
3. Click Unsubscribe in the line of the Master you want to unsubscribe from. This operation takes several seconds.
4. Refresh the page to check. Status of subscription should be changed to Unsubscribed.
You could always renew your subscription to this master by clicking Activate in the line with the Master you want to subscribe again.