I think it's best to contact them directly about this. What I have seen on their site is that you need to fill in a Service/Feature Cancellation Form and send it back to them. Good luck with it.
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.3 Client may terminate this Agreement at any time for any or no reason upon written notice (via fax, mail, or online cancellation form) to BurstNET™, within twenty-four (24) hours prior or following end of current service term. BurstNET™ may terminate this Agreement at any time for any or no reason upon fifteen (15) days written notice (via fax, mail, or email) to Client, prior to end of current service term. Notwithstanding the foregoing, BurstNET™ may immediately block Client's site or immediately terminate Client's access to and use of the BurstNET™ Service and software; if at BurstNET's™ sole discretion, it deems any information contained in Client's site to violate BurstNET's™ Terms of Service (TOS), BurstNET's™ Basic Policy and Service Guidelines (AUP), or to be otherwise objectionable or offensive or to violate the law, in accordance with Section 4.6 herein. Client's only right with respect to any dissatisfaction with any terms, rules, policies, guidelines, or practices of BurstNET™ in operating the BurstNET™ Service, any change in the content of the BurstNET™ Service, or any change in the amount or type of fees charged in connection with the BurstNET™ Service, is to terminate this Agreement by delivering notice to BurstNET, effective the day BurstNET™ receives notification of termination or any future date specified which is acceptable to BurstNET™. BurstNET™ may terminate immediately any Client who misuses or fails to abide by this Agreement, BurstNET's™ Terms of Service (TOS), or BurstNET's™ Basic Policy and Service Guidelines (AUP) BurstNET™ may terminate without notice Client's access to and use of the BurstNET™ Service and Software upon a breach of this Agreement. If BurstNET™ denies you access to the BurstNET™ Services, you shall have no right to obtain any credit(s) otherwise due to you. BurstNET™ shall have no responsibility to notify third-party providers/users of services, merchandise, or information, associated with your BurstNET™ Service, nor shall BurstNET™ have any responsibility for any damages that result from the lack of such notification.