I've found that it's actually pretty easy to cancel SiriusXM service--if you stop paying for it, they'll eventually stop giving it to you.
It's funny how their terms of service say if you subscribe online you can cancel online, but apparently if you originally didn't subscribe online (say, because your vehicle came with six months of service), let your subscription lapse, then resubscribed online, they don't consider that subscribing online, because they don't give me an option to cancel online. As a result, I use a virtual credit card with a super low monthly limit, so when they jack the rate at the end of the promotional period, the charge is automatically declined.