ADMIN PRIVILEGES REQUIRED

On the Users and permissions page found under Admin settings, site admins have a couple of options available to them for deleting a user. Click on the Actions drop-down for a specific user to access.
Delete User Deleting a user means that the user's account is deleted, and all of their content is now attributed to "User123" or a "Community" user instead.
Delete user and their content Deleting a user, and their content will remove the user, as well as all the questions, answers, comments, and articles created by the user.
NOTE: Both of the options for deleting users are permanent and can't be undone. Once you confirm your selection, the user (and, optionally, their content) is gone.