You do not have to call Comcast. If you do you will be told the following.
If you have a secondary user account you can make that the primary account, which can then delete other accounts.
If you don't have a secondary account you will have to create one then make it the primary account.
To make a secondary account the new primary account, follow the instructions in Section 2 here to unrestrict the seconday account.
After you have granted the account unrestricted status follow these directions to change account ownership
If you want to get rid of it simply because you don't want ANY Comcast accounts you will not be able to do so. You must have at least one user account.
It sounds like you want to ensure you no longer want to receive emails at the current address or any comcast address. One of the forum members is correct in stating you need at least one username for the account. But the work around I did is the following:
1. Log in and go to "My Account" "Users and Preferences".
2. Under the "Basic Settings" tab you need to add a second contact email and set it as the "Preferred". Mine happens to be a gmail account. This is required so they can contact you regarding billing issues.
3. Back at the top of that page, change your username to a different one. You need to keep track of this so you can still access your account for billing or other inquiries.
4. The system will force a new log on, using the new username. This new user name has now become your new email address. Therefore, if you try to send an email to your old address, it will bounce back as undeliverable. And if you only want to use this username as an account username, not an email address, just don't give it to anyone!