If you want to delete a User from the system, this can be done from the User Directory. Press the Settings icon on the top right of the screen, and select Administration. From the Users drop down menu on the top of the screen, select All Users. For the User that you want to delete, go to the icon on the far right of the screen. A drop down menu will open, with the option to delete user.
Press this icon to delete the user from the system. Once a User has been deleted, Items that were assigned to that User still appear as normal until the person responsible for those items are changed.