So I'm using my linux desktop for many thing including a media center. It's a multiscreen setup with a lcd monitor and a TV. I'm looking for a way to disable just a single screen when I'm watching movies.

`xset dpms force on` does turn of both screens - maybe there is an option to disable just one?

xrandr --output HDMI-1 --off

Turning it back on is less simple, as there's no --on option. One way might be to log out, then back in, if logging out restarts X. xrandr --output HDMI-1 --auto will turn it back on, but not necessarily where you expect, which may result in it being a mirror of the other display. Something like xrandr --output HDMI-1 --auto --primary --left-of HDMI-2 should suffice. You'll probably want to set up hotkeys to run these as mini scripts, or set them up as aliases.

This may seem very non-technical, but can't you just turn off the monitor when you aren't wanting it to show anything?

FWIW, I'm using arandr for this. Very simple, intuitive GUI frontend to xrandr.
It is also possible to define keyboard shortcuts that execute the appropriate xrandr commands.

Turn the switch off on the device you want off.

The problem with the suggestions to just turn off the other monitors is that the virtual screen size is still expanded to them. So the windows, the mouse cursor, some games, login windows, etc, all tends to disappear in the extra turned-off monitors. At least, it does that on my computer. In Slackware's XFCE, I'm able to press 'Windows/Super-P' shortcut and choose the preferred monitor setup. I have a two monitors set up and I can turn on and off the second one this way.

If that shortcut doesn't works for you, try entering this in your terminal as a normal user. This is what the 'Windows/Super-P' shortcut in XFCE is mapped to.