To turn off globally (can be overridden by user), I created:
/etc/xdg/xfce4/xfconf/xfce-perchannel-xml/xfwm4.xml with this added:
<?xml version="1.0" encoding="UTF-8"?>
<channel name="xfwm4" version="1.0">
<property name="general" type="empty">
<property name="use_compositing" type="bool" value="false"/>
</property>
</channel>
I tested this and it works for me.
But only for the users that make the first login after this configuration is done. For the users that already made the first login, it seems like the old settings have been cached somehow and this does not fix it.
However it should be possible to write a simple script that edits `.config/xfce4/xfconf/xfce-perchannel-xml/xfwm4.xml` on each user account and modifies this setting ("use_compositing").