Graffiti is the wilful defacing of other peoples property. it is no differant from throwing a brick through your neighbour's window.It is against the law.
Too many people treat it as "art" and " well insurence will cover the removal of it" True Insurence will cover that, now ask the question "Why is insurence so expensive."
If these people want to express themselves why not put it on Canvas or chipboard and Display it out the front of their place. who know, the talented ones might even be able to sell some of their work.
Being a cleaning contractor who has to remove Graffiti I have noticed that these people do not do their work on their own property.
Having said all that, Once In awhile you do get to see something quite witty.
At a service station where i worked, in the toilets they had a bank of Condom dispencing machines.
Right across the face of these machines some one had written " My Daddy says these things (Condoms) don't work!"
I guess that says it all in more ways! Then one think on it!