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On GitHub, 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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To disable pull requests entirely, under "Features", deselect Pull requests.
If you decide to enable pull requests again in the future, any pull requests that were previously added will be available.
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To restrict pull requests to collaborators only, in the dropdown under "Pull requests" select Collaborators only.
In personal repositories, a collaborator is anyone who has been invited to the repository. For more information, see Permission levels for a personal account repository.
In organization repositories, a collaborator is a user who has been granted the write, maintain, or admin role for the repository. For more information about organization roles, see Repository roles for an organization.