how do I remove a startup application from the command line?

Quoting the Xfce Docs ("Some of my applications are always started when I login"):

You can also manually delete those files in ~/Desktop/Autostart and ~/.config/autostart.

For example: open a terminal, type

cd ~/.config/autostart
ls

then rm the files that you want to delete.

You can disable autostart entries by hiding them with a higher-precedence entry.

If you don't have it yet, create your own autostart folder:

mkdir -p ~/.config/autostart

and then create .desktop files matching the names of the ones you want to disable from /etc/xdg/autostart (or other sources such as /usr/share/gnome/autostart).

For example, disabling print-applet:

cat > ~/.config/autostart/print-applet.desktop <<EOF
[Desktop Entry]
Type=Application
Hidden=true
EOF