"But I'm not sure I buy that one is less violent or more ok just because it has been paid for."

Totally agree. A good point. Now I'm getting all confused about the whole idea of public property in general.

Anyway, when it comes to spacial resistance, this project (http://www.goodmagazine.com/blog/orions_reverse_graffiti) flipped what most people find objectionable about graffiti on its head.