Eu encontrei esses métodos nos Fóruns do Ubuntu em um tópico intitulado: Tópico: Como bloquear a tela no XFCE? .
extraído de 2 das respostas nesse tópico
Método 1 - atalho de teclado
Open the settings manager > keyboard > shortcuts and you can see that the default shortcut to lock the screen is ctrl-alt-del. If you want to change it, click add on the left, type in a name for your list of shortcuts, (widen the window so you can see the whole thing) select xflock4 shortcut on the right and enter the new key combo.
Método # 2 - via linha de comando
$ xflock4
Método # 3 - xscreenlock
Na maioria das vezes eu uso xscreenlock em uma infinidade de distribuições Linux. É bastante onipresente.
trecho do site dos desenvolvedores
XScreenSaver is the standard screen saver collection shipped on most Linux and Unix systems running the X11 Window System. I released the first version in 1992. I ported it to MacOS X in 2006, and to iOS in 2012.
On X11 systems, XScreenSaver is two things: it is both a large collection of screen savers; and it is also the framework for blanking and locking the screen.
On MacOS systems, these screen savers work with the usual MacOS screen saving framework (X11 is not required).
On iOS devices, it is an application that lets you run each of the demo modes manually.
imagem da janela principal
HáumatoneladadescreenshotsdosváriosscreensaverseoXscreensavertambémoferecebloqueiodetela.