Note: you must be the Administrator of your email domain in order to add/remove users. If you're not sure, follow the steps below and if you see an Admin button inside your webmail then you are an Administrator.
To delete an email account, log in to your webmail account at http://mail.[yourdomain]. (For example if your domain is abc123.com then you would log in to http://mail.abc123.com)
Once logged in, click on the Admin button on the left side navigation
Then click on the Users link.
This will bring up a list of users. Click the checkbox next to the email account(s) you want to delete.
Click the Delete Selected button.
Note: this will completely delete the user and all emails / data in their email account, and is not reversible.
A confirmation message asks you to confirm that you want to delete the mailbox. Click Ok to confirm the deletion.