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 your GitHub Enterprise Server instance, navigate to the main page of the repository.
Under your repository name, click Settings.
In the left sidebar, click Collaborators & teams.
Next to the collaborator you want to remove, click the X icon.