Hi Alba De Castro. I believe you registered to Grokker through Roku. Therefore, you will need to cancel your subscription from your Roku device. Here are the instructions on how to do that:
1. Highlight the app tile on your Roku screen
2. Press the asterisk (*) button your Roku remote
3. Navigate to Manage Subscription, select this
4. From this menu, click Cancel Subscription
If you need any additional support from Roku, you may call their customer support number at 1 (816) 272-8106.