A story from the Daily Telegraph in 2007:
Each gang is ethnically based and routinely target teens for mobile phones, shoes and popular clothing and are involved in large scale graffiti, drug selling, vandalism and assorted robberies.
Nice kids, obviously.
Not everyone who sprays graffiti is a nasty little criminal, and not every criminal runs around spraying graffiti in their wake. However, it's interesting to see how often graffiti goes hand in hand with "harder" criminal activity.
I'd like the Police to pay more attention to graffiti because it provides them with a possible opening into other, possibly worse, criminal activity.
When the New York Police started enforcing zero tolerance, they arrested a bloke for jumping the turnstiles at the subway and after finger printing him, found that he was responsible for a string of murders. Criminals commit all sorts of criminal acts; busting them for the low range crimes may in turn lead Police to detecting other activities which are viewed much more seriously.
In addition, let's say that a particular criminal is involved in all the activities listed by the Daily Telegraph; graffiti, robbery, drug dealing and assault. The harder offenses may be harder to prove, but getting them locked up for a low level offense like graffiti still has the effect of getting them off the street. Al Capone went to prison for tax evasion, not murder. Take the same approach with graffiti.
As for cleaning it off, here is a story from the Village Voice from 17 July on graffiti. This is the photo used in the story, showing a wall being cleaned.
Intrigued by the story, I visited the site on 3 August. I'm not entirely sure what they cleaned up.