Esta pergunta é respondida na wiki do Xfce, subseção Alguns dos meus aplicativos são sempre iniciados quando eu faço o login :
There are two possible reasons why the application is started: It is saved in the last session or it is listed in the auto started applications. Follow 1 of the two steps below to get rid of the applications.
Start the xfce4-autostart-editor and remove the application(s). You can also manually delete those files in ~/Desktop/Autostart and ~/.config/autostart.
Most of the time closing all the applications and save your session when you logout is sufficient. If this doesn't work, remove the content of the ~/.cache/sessions/ directory when you're not logged in. And if you don't want xfce remember every session you should turn off (uncheck) “Automatically save session on logout” in Settings Manager → Sessions and Startup (tab General)
Execute isto:
rm -fr ~/.cache/sessions/*
e o Xfce deve começar de forma limpa.
Além disso, talvez seja necessário limpar todo o diretório .cache
se o acima não corrigir os problemas:
rm -fr ~/.cache