How can I cancel my billing subscription?

You can view and manage your subscriptions via Paypal. Once logged into Paypal click on Profile (Wheel icon next to the logout button, -> "Pre-approved payments" under "Payment settings".
Here you will see all of your pre-approved payments, click on the Merchant "MumbleMe" and this will take you to another page which describes the details of the MumbleMe subscription.
If you wish to cancel your service then click on the "cancel" link.