I've had Comcast service for five years, and it has never worked well. But, there is a way to get things done, and it's to use their phone-tree to get to the 'disconnect' department (also known as the 'loyalty' department).
They have the override-capability which other departments, including the executive offices, do not. They can also arrange for account-credits, when other departments will not, or cannot.
But, you have to go through the Comcast phone-tree CAREFULLY, because the disconnect department is usually hidden away under some category like 'other.' Sometimes you'll also hear the phrase, 'do you want to downgrade or disconnect service?' That works, too. Good luck!