It is said that gypsies marked homes which were a soft touch. A system of marking for different reasons is an interesting idea. We can develop a code, shrouded in mystery...revealed in incremental layers to those who swear a sacred oath. Just a simple coloured dot from a can of marking paint in just the right spot (for example) will warn all that a particular "customer" doesn't pay their bills. Before long we will all have the pleasure of watching them mow their own grass for a change...as we drive by and wave. But woe to the one who dares reveal the sacred code. Darkness and doom be upon them for three generations.