Set Desktop Wallpaper And Prevent Users Changing It?

I've tried a few things to get around this, some kids where I work keep changing the wallpaper, sometimes to quite disgusting things. Since groups of them all use the same login with roaming profiles, everyone gets to look and laugh at the annoying wallpaper.

I've tried enabling active desktop local policy and preventing users changing wallpaper but as far as I could tell active desktop didn't even come on, I refereshed the policy.

Does anyone know a good efficient way of simply setting a wallpaper for all users in an ad ou and preventing them changing it? I've got rid of all ways to set a desktop wallpaper apart from the right click menu on an image which has the "set as backgound" option, that's what they are using to do it.

I don't really mind how it's done, could be a registry hack to remove that option from the right click context menu when clicking on an image, I could then deploy it with group policy.