Recently I found myself in need of a way to switch X into single-monitor mode without having to kill the running instance of X, which is fine if you don't have any unsaved work, since it crashes all programs running within X as well.

xrandr

Using xrandr[^1] we are able to dynamically modify our desktop's properties.

OFF

1

xrandr --output DVI-0 --off

ON

1

xrandr --output DVI-0 --left-of VGA-0 --auto

[^1]: Xrandr's Man Page