Users can now unsubscribe from all repositories owned by a given user or organization. Navigate to github.com/watching to find a list of the repositories that you are subscribed to. The Unwatch All button gives you the option to unsubscribe from all repositories, or just the ones that belong to a specific user or owner. It will appear when you are watching all activity or custom notifications on over 10 repositories.

Clicking any of the dropdown options opens a modal to confirm unwatching repositories owned by the selected user/organization (or all repositories).

You can read more about how to unwatch a single or multiple repositories in the 'Managing your subscriptions' documentation