To remove a user from your organization, revoke their membership and delete the user's forks of the organization's repositories.
Warning: When you remove a user from your organization, any forks they have of your organization's private repositories are automatically deleted.
When you remove a user from your organization, they lose access to the organization's private repositories and settings. They will retain read-only access to any public repositories, but will lose any write or admin access they might have had.
If you're not already signed in to GitHub, sign in to your personal user account.
On the Organizations Settings page, at the bottom of the settings sidebar, choose the organization.
In the left sidebar, click Members.
Next to the name of the users you want to remove from the organization, click the check box.
At the top of the page, under the organization name, click Remove from organization.