Cancelling Premium or stopping future charges is handled through whichever app store you subscribed with, because the billing runs directly through Apple's App Store or Google Play rather than through Vagustim. To stop renewals, you will cancel within your store account's subscription settings, and that is what prevents any further charges.
If you would like a hand making sure everything is set correctly on the account side, support is glad to check. It is also worth remembering that your device purchase is entirely separate from your app subscription, so cancelling one does not touch the other.