How to solve your problem and more forever:
Setup a new user. I called mine "steam".
Install openbox. It's small and harmless: "sudo apt install openbox"
You can now select openbox as a login. It is an EXTREMELY lightweight session that opens only to a blank screen. You can right click the blank screen to get a terminal if you need it.
From the terminal or by logging in as that user normally, install steam for that user.
Find the name of the secondary monitor you don't care about by typing "xrandr" and figuring it out from the connector. If you don't have xrandr (I think you will) then "sudo apt install xrandr"
Create and edit a file "/home/steam/.config/openbox/autostart" (Replacing "steam" with whatever you named your user).
Put the following in the startup:
xrandr --output [your monitor] --off
Replacing your monitor with the one you don't want. Mine was "DVI-I-1" for example. If you're not sure, try the command from a terminal. It won't save it if you screw up, you can just log out.
Now when you want to play games you login as "steam" (or whatever) using the openbox login. Your secondary monitor is shut off and never a problem with anything. Additionally, you have no openGL compositor which can sometimes cause problems, and your system is running extremely lightweight using only 300 or 400 meg or so of RAM.