This is possible from within the Hullomail app via the following:
Menu -> About -> Service Termination.
Once the service has been terminated, please remove the app from your handset.
On Android, you'll be automatically setup to forward calls back to your carrier voicemail.
If you use an iPhone, please dial the contact 'Hullomail Deactivate' to forward calls back to your carrier voicemail, or see FAQ: "How do I Deactivate Hullomail?"
Please remember to cancel your Hullomail subscription to stop renewal, or see FAQ: "Cancel a Recurring Subscription"