Debited every month for service I did not use

I keep getting debited even though I only used the service for one month. There is no option to cancel the 4G internet and you people continue to debit me, costing me a lot of debit order charges for an insufficient amount.

How do I cancel my account so that you people can stop debiting me every month? I only meant to use the service once but I'm being billed every month!