This time I called but they didn’t actually cancel the service so I had to call again when the monthly charge appeared on the credit card. They could see my first cancelation call in their records, and where appropriately apologetic etc. Though they didn’t do anything to compensate for the mistake, they did reverse the charge.
Like most such services they try very hard to make it difficult to find the phone number to cancel. So I’ll post it here, so I can find it next time: tel:+1-800-880-4117; as can others. The trick to finding it on their site is to search for terms and conditions, and then look for 800 in the fine print.