If you're using grub to boot, identify the screen name and enable only the screen you want to keep from grub parameter in adding to variable LINUX_CMD_LINE_DEFAULT: "video=VGA-1:d" or "video=LVDS-1:d" or ... depending on the screen name.

You may find this variable in /etc/default/grub
You may find the screen name with:
xrandr -q

and don't forget to upgrade grub config with something like grub-mkconfig

If you're using grub to boot, identify the screen name and enable only the screen you want to keep from grub parameter in adding to variable LINUX_CMD_LINE_DEFAULT:

video=VGA-1:d or video=LVDS-1:d

or ... depending on the screen name.

You may find this variable in /etc/default/grub

You may find the screen name with:
xrandr -q

and don't forget to upgrade grub config with something like grub-mkconfig