If you used a credit card to activate the account, you're being charged for it. The service only stops until you tell it to, otherwise, it's assumed you want to continue.
There's also a bug in the XM system. If you terminate your service, a cancel signal will be sent for 30 days. After those 30 days, the signal stops.
Assuming you haven't turned on your tuner, to accept the stop signal, it should never be deactivated. I haven't tried this, but I'll let you know how it goes soon.
I nuked my service back in December and will try to turn on my radio for the first time since, in April.