You buy a card with a unique id that grants access to any public bathroom cabin. Anyone with such card can report a cabin if it's a mess. If it is - the previous visitor's card gets banned. You also get banned for false reports. The maintainers of the bathroom verify the reports. The card needs to be expensive enough to not be disposable, but also cheap enough so that all social classes can afford it. Problem solved without any need for any private data, etc