Refusal to cancel an insurance policy despite the vehicle being sold

My mom was involved in a serious accident 5 months ago which resulted in her being a quadriplegic, hospitalised and unable to move from her neck down. We had to sell her car but despite calls, e-mails (from her registered e-mail address), proof of sale (from We Buy Cars), proof of my mom's condition from the hospital, Discovery Insure still take the premium off her account & insists she calls the call centre in person to cancel. The are quick to sign you up and not quick to cancel insurance. It actually theft!