Here is how to cancel an automatic payment with a merchant:
Click the Settings icon next to “Log out.”
Click the Payments tab, then click Manage pre-aproved payments under “Pre-approved payments.”
Select the payment you'd like to cancel, then click Cancel.
A subscription can be cancelled up until the day before the next scheduled payment in order for you not to be charged.
How to cancel the subscription from the Play store:
To cancel the auto-renewal for your subscription:
Apple has an excellent knowledge base article all about purchasing and managing auto-renewing subscriptions: http://support.apple.com/kb/ht4098