In order to delete a user, go to User - Management --> User - Overview. You will see a list of all users available.
Fig. 5.2.3 - Delete a user
Click the 
-button next to the user you want to delete. After confirming the security-prompt the user will be deleted.
PLEASE NOTE: It is not possible to delete the administrator. There is no -button for this user. You are only able to modify this user using the 
-button.