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
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On GitHub, navigate to the main page of the repository.
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Under your repository name, click Settings.
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In the left sidebar, click Manage access.
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To the right of the collaborator you want to remove, click .