Systemtray and wallpaper protection

I’m an administrator of cybercafe. The computers are used by various people (children and unfortunatelly porn addicts). We make everyday backup but it prevents from offensive content only partially. During a day kids are offended with xrated wallpapers and tray dialers, so I’m asking – Is there any way to protect system tray not to allow installing programs there and is there a way to protect from changing wallpapers without using any external programs? Any way to writeprotect seperate keys in registry?