Log in with your ZelloWork network credentials and click the "Billing" tab.
Choose the "Please cancel my subscription" link.
Please note that this is permanent, so you might prefer to downgrade to the free service plan. You can do this by entering 5 users and 0 gateways. Make sure that the subscription fee is 0 USD and then press "Continue". You will see a message that says "Changing service plan", and you will not have any more charges for your account.