How can I cancel my subscription?
You can cancel your Firewall subscription at anytime!
All in-app purchases are handled by the App Store, so you can cancel your subscription through your iTunes account.
Here is the support article from Apple, it's essentially the same but they include screenshots: https://support.apple.com/en-us/HT202039
Note: Make sure to disable forwarding before cancelling your subscription. Otherwise your calls will continue forwarding to a Firewall number.