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 your GitHub Enterprise Server instance, navigate to the main page of the repository. 
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Under your repository name, click Settings. If you cannot see the "Settings" tab, select the dropdown menu, then click Settings.  
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In the "Access" section of the sidebar, click Collaborators & teams. 
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To the right of the collaborator you want to remove, click Remove.