Log-in through the xfinity website, go to users and you can remove the secondary account or contact customer service and ask help from them to remove the email account.
Added an additional email account. Want to delete it. How do I do it?
Sign into your Primary account, then on the landing page click on Users at the top. You should see the list of accounts you have. To the right click on Edit. On the next page, under the account name, you should see the options to Suspend or Remove the account.