This is possible from within the Hullomail app.
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 to stop Hullomail answering my calls?"
Please remember to cancel your Hullomail subscription to stop renewal, see FAQ:
"How do I cancel a recurring subscription?"