This is an un-authenticated method of communication. It might be hard to stop people from using this, but it seems easy to overwhelm them with misinformation.

This is an un-authenticated method of communication. It might be hard to stop people from using this, but it seems easy to overwhelm them with misinformation. Start placing 10, or a hundred times as many of these signs around non-important objects. Drown the signal in noise. Some consideration is needed for how to keep these symbols believable, without accidentally marking certain people for being bombed. Still, I think if you have marked 60% of roofs in a town with these signs, then some-one will realize something is wrong.