ANSWER:

Seems like the TV as a display is being detected even after being switched off. Just pulling out the dvi cable, if it is easily accessible from your seat, should work

If it isn’t, then use the following command to turn of the TV as a monitor.

xrandr --output XXX --mode HRESxVRES --output YYY --off

Here XXX is the name of your primary display. YYY is the name of your TV. HRESxVRES is the resolution you want to set your primary monitor to.

To figure out what XXX and YYY are, inspect the output of the command xrandr --current when both monitors are on and being used.

XXX will most probably either be LVDS1 or VGA1 and YYY is most probably DVI1.

In addition, the output will also contain the allowed values for HRESxVRES.

You may choose to add an alias for the command or add a launcher button that executes the command.

S Prasanth from http://askubuntu.com/questions/297692