Cancellation of service which they refuse to accept

I have a problem with Mweb billing me for service I didnt use , I paid for signing up and after 6 weeks of no communication they delivered a router. I expressed how I was no longer interested and told them to take it back , which they did but now they keep saying I need to pay them. I will not pay for installation or any service I didn’t use . Now they sending threats with letter of demands for outstanding fees. I keep calling and they don’t take my calls