Or... You could just say two words: "Too Expensive". Comcast asked me why I cancelled my service 2+ years ago, and that's the answer I gave them. Apparently, they believed me, and I was a comcast customer for longer than 9 years before I cancelled.
Now here is the interesting part: Ryan Block is an internet celebrity, and his wife is Veronica Belmont, also an internet celebrity. I would not be surprised if Comcast had a note of this in his file, and that is why the rep was so adamant about getting a good reason from him. It's almost like Aquaman calling the local water works company and telling them that he doesn't want water anymore. They wouldn't believe him.