[ubuntu] Prevent users to change wallpaper

Hello, I am new in this forums, and my english level is not the highest it could be (I am spanish)

I have been trying to prevent users to change their desktop's image on Ubuntu 16.04, but I was unable. I heard that editing /usr/share/unity-2d/shell/launcher/Launcher.qml and deleting all strings that contains "wallpaper" I could make it, but the directory does not exist anymore. Also I try changing Desktop/ permissions, but neither it works. Using dconf-editor seems to have the solution to my problem, but in org.gnome.desktop.background I did not see the way I can lock the wallpaper.

Hope you can help me, and again sorry for committing grammar mistakes or misspellings.