If you mean your primary account----------you can't. It's your login credential for the entire Comcast account. If you don't want to use it just don't. If you mean a secondary account you created then--------------------
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.
I No Longer have Comcast .. I want to Delete My Primary Acct. and I find no way of doing it. .. I Want the primary E-Mail Acct Deleted .. Someone better figure out how to do it.. They certainly have enough 'bread' ($) ..and keep asking for more.. that they can find a way so people can delete their primary accts once they leave this 'real classy establishment'
In that case, you could call Comcast security and see if they can do that-------no guarantees.
Comcast Customer Security Assurance-------------------
Normal business hours (6:00 am to 2:00 am EST, 7 days a week)
1 - 888-565-4329
But keep in mind that if you simply stop using it, the email address will vanish after 9 months without you signing into it. I would just abandon it.