To delete an existing user go to the left side menu and click on "Edit Users". Select the user you want to delete by selecting the respective radio box. Note, that the original default user cannot be deleted, since there needs to be at least one user to be able to log into the server/domain. Click on "Delete Selected Users" in the end.
Confirm the deletion process by clicking on "Yes, Delete Them".
Only the default user should be listed now.