Anyone with Administrator access can manage their User list and delete. If you're the Administrator, click on your name up in the right hand corner of the main page and then on Users from the drop down menu.
The Users page will open up with a list of everyone you have added as a user with their information. Each user's name is actually a hyper link indicated in blue. Click on the User's name you wish to delete.
The User's detailed information will be shown next, and off to the right in the Action Panel click on the Delete User icon.
If your user has been assigned any tasks that they have yet to complete, you will need to re-assign each task to another writer before proceeding and deleting your user.