To completely remove a user from your organization, you must do two things: revoke their membership and delete the user's forks of the organization's repositories.
When you remove a user from your organization, they lose access to all the organization's private repositories and settings. They will retain read-only access to any public repositories, but will lose any write and admin access they might have had.
Sign in to your personal account
Go to your organization's main page.
YOUR_ORG_NAMEwith your actual organization name.
Click the Organization settings gear next to your organizations name.
Click Members on the right side of the page.
Select any users you want to remove from the organization.
Click the Remove from organization button at the top of the page.
Warning: When you remove a user from your organization, any forks they have of your private repositories are automatically deleted.