Jason Waters
I wouldn't be surprised if they keep billing you. I had this same situation a few years ago (when it wasn't $300 either) and I followed up with an email to their customer service, saying I had spoken to XXXXX and my service would be cancelled. My radios kept playing and I got another bill in a month (yes, I had paper billing, which was like an extra $2 or something), saying I now owed for two months. I refuted it, and they said they had no documentation, so I said I would forward them the email, again in email. The second time around, I made it clear they would not get their money, and referenced the first email where I documented my cancellation. While they tried to tell me I could only cancel by phone, I said I did cancel by phone, and documented it in an email to XM. That way I had something to give a credit agency if there was ever a problem. Service ended that night.

They remind me of the cable and phone company before there was competition. Unfortunately, some people still have only one option.