Hallo OppaErich,
KDE can save its config as default.
If your desktop can't do this, you can create your config at startup.
xrandr can disable a monitor on the fly and you can script it.
I would prefer to add an ignore option like this to my /etc/X11/xorg.conf.d/dualhead.conf:
# /etc/X11/xorg.conf.d/dualhead.conf
# Identifiers taken from xrandr
Section "Monitor"
Identifier "DVI-I-1"
EndSection
Section "Monitor"
Identifier "HDMI-0"
Option "RightOf" "DVI-I-1"
Option "ignore" "true"
EndSection
# Ende dualhead.conf
greetings
musca