Worst experience tryin to cancel the service!

Typically, extremely efficient when it comes to sign ups but the after sales service is terrible. My elderly parents cancelled 3months ago and are always promised to receive the cancellation confirmation letter after numerous phone calls to the inefficient call centre - up until today, no letter! They have been billed for last 3 months and not even using the router / wifi. Seems like the only way to get this resolved is to change bank cards and report the ongoing transactions as ***** - best to keep all the cancellation request documents / emails on hand when this matter goes the legal route.