Cant cancel account

Im trying to cancel my month to month lte contract, because MTN decided to change the fair use policy from 10mbs to 2mbs. Now, mtn wont except the modem because i dont have the box or grey cable the modem came in... if mtn told me i would need to keep the box i would of.... so now the store wont except the modem and the customer care keep transfering me cancellations, and cancellations keep tramsfering me back to customer care... if MTN told me to keep the box and grey cable i would of... how do i cancel this account... ***