As far as I know you can disable it using “Toshiba HW set-up” utility.
On desktop click on “Toshiba Assist” (I hope you have it preinstalled on your notebook) > Optimize > Toshiba HW setup > check if tab “illumination” is available.
If yes use option OFF and confirm with apply.
This should help you.
You can also check advanced BIOS settings. If I am remembering well this illumination option can be also disabled there.