I can't claim credit for the idea, but I think that the 'paper tag' (like you might find on clothing in a store) with the appropriate instruction. It's cheap,

I can't claim credit for the idea, but I think that the 'paper tag' (like you might find on clothing in a store) with the appropriate instruction. It's cheap, it will annoy people until they remove it, and it's fairly obviously a sign that it needs removing.