Bad service from Sonic Herotel

I advised them on the 11th June 2020 of the intention to cancel my internet service which I pay for in advance on a monthly basis. They sent me an invoice for the next month later in June at which time I reiterated my intention to cancel. They eventually rep**** stating that they require a month notification. I advised that I have I formed them on the 11th June to which no further communication was received. The month notification would then end on the 11th July 2020 as per their policy. I was in credit which would cover the pro rata amount due. I have been threatened with my record being handed over as a defaulter for their terrible service and dealing with the cancellation