At the moment, it's not possible to terminate a user that hasn't started working yet, which means that you won't be able to terminate a Hired employee. You'll only be able to terminate them once they have reached their start date and are marked as Employed.
We do have a workaround for this, should you not want that this employee's "first day" to affect your dashboards. You may change the employee's first date far ahead in the future, which will leave their account as "Hired" and therefore not affect the Dashboards or the Attrition attribute.
You may also permanently delete the employee account, but please note that this is irreversible - to delete an employee, please navigate to the employee's profile and click on Actions > Manage access > Delete Employee's File:
! Please note that anything that is deleted cannot be recovered !