Here you can have the steps to block the external contacts in different ways in Skype for Business through the Office 365 Admin Center.
You’ll learn updated features to the SharePoint Modern admin center in office 365 and how we can navigate from old admin center. Here I’ll also given my comments need to improve in admin center.
Shows you the steps to turn off the external sharing options to all SharePoint Online Site Collections and OneDrive for Business for all members
Simple steps to disable the sharing option to outside of your organization to restrict the contents in SharePoint online.