Refusing to cancel policy

I took a joint insurance on behalf of myself and ex-husband to cover the bond we had taken from the house we bought together. The house is now being sold and I am requesting the cancellation of the insurance but they are refusing, stating that they require written letter from my ex-husband because his he was set as the primary insured and they have been emailing statement to him. I am the one who took insurance, with my contact details and have been paying for it all these years. Why can't I be allowed to cancel it? Also they took over a week to respond, delaying me. I don't want to pay for something I no longer need