The refund from Comcast has finally arrived today. I still haven't received my final statement by mail but Sandi who followed up by email to make sure that I received the check was nice enough to email the statement to me. So, it seems that I was able to cancel my service with Comcast after all. It was a very painful process and I hope that other Comcast customers are treated better than me but at least I'm done.
I should write another post later that summarizes all the advice I got about this issue. But I can say quickly that complaining publicly is much better than trying to talk to customer service of Comcast because (at least in my experience) their customer service reps are not capable of doing something as simple as closing an account even after you talk to them month after month for 5 months.
So, if you're not having luck with the Comcast customer service, try @ComcastCares on Twitter and if at all possible, get a story on Consumerist . This approach worked for me.