User Configuration\Administrative Templates\Windows Components\Internet Explorer\Browser menus
Prevents users from viewing the HTML source of Web pages by clicking the Source command on the View menu.
If you enable this policy, the Source command on the View menu will appear dimmed.
If you disable this policy or do not configure it, then users can view the HTML source of Web pages from the browser View menu.
Caution
This policy does not prevent users from viewing the HTML source of a Web page by right-clicking a Web page to open the shortcut menu, and then clicking View Source. To prevent users from viewing the HTML source of a Web page from the shortcut menu, set the Disable context menu policy, which disables the entire shortcut menu.