If you mean the primary account----------you can't. It is the sign-in credential for the entire Comcast account. Just stop using the email if you don't want or need it. If you are talking about a secondary account------------
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.