Deleting forks of private repositories
While forks of private repositories are deleted when a collaborator is removed, the person will still retain any local clones of your repository.
Removing collaborator permissions from a person contributing to a repository
On GitHub.com, navigate to the main page of the repository.
Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.
In the "Access" section of the sidebar, click Collaborators & teams.
To the right of the collaborator you want to remove, click Remove.