How can I prevent users from changing the desktop wallpaper on Ubuntu 18.04?
You can't prevent users from changing your desktop background without preventing your own account from making a change. If they are using your account, they have the same access that you have.
Give them their own separate login ID. This way your environment will never be affected their settings.
Activate a script that will check for a change and periodically change it back to your preferred background.
You can find an example of such a script in this answer:
How can I get one wallpaper for each day of the week?
Instead of having the script change daily, just have it periodically set the background to your choice.
Yes you can. You need to enforce dconf read-only lock on the background property. I assume you use gnome shell or any destop than use dconf.
So create a file
/etc/dconf/profile/user with the content
This defines a user dconf db -which is always defined anyway, but also define a system db that we will use to lock the key we want.
Now create the directory
/etc/dconf/db/local.d. This is where reside the keyfile you want to set.
But to lock key, you need to create also
/etc/dconf/db/local.d/locks directory. now in that directory any file with a key will be locked.
/etc/dconf/db/local.d/locks/00_wallpaper with the content:
# prevent changes to the wallpaper /org/gnome/desktop/background/picture-uri
sudo dconf update
Then users can't change the wallpaper anymore !