This is a remix of Light Switch Lock by Yllonnoce on Thingiverse. I've removed the “no” symbol and removed the plastic behind the symbol to simplify it as much as possible.

This design prevents flipping a light switch off, which is useful when using smart bulbs like Phillips Hue or any other cases where you want a switch left on. The switch can still be flipped by just gently pushing the arm out of the way. It can also be mounted upside down to keep a switch in the off position.

My compliments to the original designer as the design is very clever! It's unobtrusive and nicely prevents flipping a switch while still being fairly easy to switch when needed.

I printed with PETG but PLA should be fine too.