I don't believe you can do it -absolutely-. Windows 8 might be very different in terms of how it looks when in the Modern UI, but the desktop, even though it's shown as just an app, is still the very base of the system. It requires it to work, and it's always there, somewhere, as illustrated by the fact that Skype (desktop) auto-starts, whether I start the desktop "app" first or not.
Alternatives I would suggest:
If you're good with that stuff, I'm sure you can design a software solution that would, on selected accounts, just pop people back to the Start screen if they should end up on the desktop. If malware can do much weirder things, I'm sure a good developer can.
For a less aggressive approach, make a pop-up appear each time the user enters the desktop: You have entered the Desktop (advanced mode). Are you sure you want to continue? You can go back anytime with the Windows key.
Perhaps there's even a way to track down all "Desktop" shortcuts so that they point back to the Start screen.
Empty the desktop from everything it could have that actually makes it look like a desktop (icons, task bar, whatever).
Then you would change the wallpaper to something you would have made yourself, looking Modern UI-like (white Segoe UI font and solid colour, simply), that would tell to go back to the Start screen. There are several ways to do that.
-Give instructions on-screen like "press the Windows key to go back to the computer" or such.
-Tell to click the screen, assuming you can manage to create an Active Desktop Web page (this tutorial for Windows 7 might just work: http://www.youtube.com/watch?v=7O1OAWMFBgY ), and to make a link to the desktop, which, not technically being a Web page, sounds hard.
-Tell to click the icon "Back to Computer". To make sure you don't confuse them, you could make the desktop shortcut with a custom icon, Modern UI-style, activate opening by simple click (Folder Options) and making the icon as huge as you can (somewhere in Display, in the control panel).