Call Comcast and tell them you want to terminate your account.
To logout of all devices on Comcast, you will need to log out of your account on the website. To do this, go to https://my.xfinity.com and click ‘Sign In’. You will then be prompted to enter your username and password. If you are unable to remember these, you can use the ‘Forgot Password’ feature by clicking ‘Lost Your Password?’ in the top right corner of the page.