I had a similar thing with Comcast which kept referring me to their retention department and asking me to call.

I replied to them no less than 5 times, each time pointing out that (a) I don't have time for phone calls (b) their terms and conditions permits cancelling by e-mail.

Another way is to notify them once by e-mail to end your subscription per [insert T&C clause and citation] and just stop paying. If they don't get payment they'll stop anyway, and if they come chasing after you, you have the notification you sent.