TIP: if you want to cancel a service or credit card or whatever without the rep trying to talk you into keeping it, this is the answer to give when they ask why you're cancelling: "It's emotional and I don't want to talk about it."
I just cancelled a MasterCard because I got it for a specific reason that makes it painful to even look at the thing. When they asked why I was cancelling, I said, "It's emotional and I don't want to talk about it," and the rep backed off! It totally cut the conversation short, so I'm going to do the same thing with Comcast. They might be the magic words.