Once you activate a free trial through iTunes, you will need to cancel before the end of the trial period to avoid being charged for the annual subscription.
To cancel, all you need to do is manage your auto-renew option through the iTunes account settings on your iPhone or iPad. It's important to note that this has to be done on your mobile device itself, and cannot be done on our end.
You can access your device's subscription settings through the Calm app 'Manage Subscription' page. Please tap the profile icon in the top corner of your home screen, then scroll past the calendar on the Profile page to the 'Settings' option. In Settings, please tap 'Manage Subscription' and on the page that appears, tap the word 'here' to be re-directed to your phone's iTunes settings. Once there, you should see the Calm subscription listed with an option to cancel.
If you have any trouble with that, please see this support article for details on how to get to the cancellation page through your phone's Settings menu: http://support.apple.com/kb/ht4098