Unlink accounts

I am an authorized user for my 84 year old parents account so I thought, incorrectly, that it would be easier to link their account to mine. There is no way to tell which account an email from Comcast is for. I have looked all over the support site and spent 30 minutes on chat being transfered to different departments. Why do you make it so simple to link accounts, yet how to unlink them is some kind of top-secret national security issue. All I want to do is unlink the two accounts. Will someone for support let me know how to do this?