Those of us not on mains drainage may have concerns with house guests. What they might flush at home could block our drains or septic tank. But it's not easy to tell them.
This little notice can sit on the cistern or stick to the wall flat, you can colour in the letters with a sharpie for extra impact. If anyone asks you can always say that OTHER guests accidentally flushed the wrong thing and you had to get the drains cleared,
Simple to print and the hinge pin is a scrap of unused 1.75 filament, it will be bent and this helps stiffen the hinge.