Poor service

When I signed up, I was made aware that the deal I took is prepaid and you pay when you want yo use the service. About 3 months ago, I changed the debit order date and paid for the service using my card but was again debit in that same month. I sent an email informing them about this and was requested to send proof, I sent my bank statement showing both amounts and that was the last time I heard from them. Last month I was away and was not gonna need the service and I therefore decided not to provide funds for them to take in my bank account, they tried to take my in my bank account almost every single causing a big chunk of bank charges then this month they have taken double the amount. What for when the service is prepaid?