If you subscribed to Grokker through the iOS App Store it is managed completely by Apple. Therefore, you must cancel your membership through Apple.
You can do this on your mobile device with the following instructions:
Open the App Store app.
Click your name. If you can't find your name, click Sign In.
Click Account Settings.
Scroll to Subscriptions, then click Manage.
Next to the subscription, click Edit.
Click Cancel Subscription.
If you need further assistance with your Apple subscription please reach out to Apple Customer Support at 1-800-275-2273.