A few months ago I showed how to disable the buttons in Gnome and KDE. Occasionally, I use Xubuntu with XFCE on older systems with limited memory available. Disabling the buttons is easier in XFCE. Simply go to Applications > Settings > Settings Manager > Sessions and Startup > General and disable the show options for both buttons.
The problem is that this setting is per-user so you have to do it for each user account individually. The settings are stored in "~/.config/xfce4-session/xfce4-session.rc". If you want to use them as the default for new accounts then copy it to the equivalent path in /etc/skel and set the ownership to root. The skel directory is used to create the default home directory structure for new users and the ownership will be changed to the user's account automatically.