An Admin user can delete a user who is no more part of your organization.
Admins can delete a user only after ensuring the following:
The user does not own any OKR.
Ensure that OKRs owned by the user is assigned to another user.
Assign all the direct reports to another user.
Is no more a manager of any team. Find another user who can replace the user as a manager.
For more information, refer to Adding the Users.