Click Start, search bar. Type GPEDIT.MSC and hit ENTER go to:
User Configuration > Administrative Templates > Windows Components > Internet Explorer > Privacy > Select "Turn off inPrivate browsing" > On the left click edit policy setting > select enabled > Ok and done!
You know whoever you're trying to stop from browsing without your knowledge can just delete their history and cookies and you'll be none the wiser, regardless of whether or not inprivate is enabled. What you really need is a filtering program to restrict access to certain web pages.