Block using Internet based Windows Update for corporate machines

Q. How can I block the option in Windows Update to Check online for updates from Microsoft Update?

A. If you leverage WSUS you would have deployed policies to the machines to point them to the WSUS instance as the source of the updates available. In the Update settings area in addition to the Check for updates button there is also a link "Check online for updates from Microsoft Update" which will bypass WSUS and instead contact the public Microsoft Update servers. To remove this option you can use Group Policy