The Xfconf daemon supports a so-called "kiosk mode" or "lockdown mode"
such that certain configuration properties or certain channels can be
locked from user modification.  In this case, the locked values are
provided by a system administrator in the form of a settings file (in
the same format as the user's settings files) in a system read-only
location.

Not all backends may support kiosk/lockdown mode.  Please see the
documentation corresponding to your configuration backend for
information on how to lock down user configuration values.

(If you're unsure which backend, you're probably using the
"Per-channel XML" backend.)
