As a admin - generally, how can/should I delete the user’s account ?
Make sure you’re an admin: https://wiki.diasporafoundation.org/FAQ_for_pod_maintainers#What_are_roles_and_how_do_I_use_them.3F_.2F_Make_yourself_an_admin_or_assign_moderators
Then you can go to the Admin area via the dropdown menu and in the user search find a user and close their account.
Thank you 