H.idden I.n V.iolence

In differentiating the sub-cultures behind graffiti, I have learned to associate artists in either groups of gang-related territory markers, or corresponding artists with messages-positive/negative-of promotion, politics, life, etc.

Here is another group of graffiti artists, however, that are taking the streets as non-gang affiliated creators, who are lashing out against the negativity of gang-related work.
(http://www.msnbc.msn.com/) provides an article on a group of individuals who mark over gang related graffiti in ways that deface their messages in hopes of embarrassing gang members and helping them in a direction of disassociation.

The article provides examples of their aims for change:

So a stealth group is secretly changing gang graffiti in an effort to make
gangsters look foolish and provoke debate among gangsters. For example, members of the group might change the
"XIV" gang tag into "HIV." SEM might be mocked as super ecological morons. VGS
might be edited to stand for violent gangsters suck.

Personally, I don’t believe any gang member would proudly be associated with HIV, or the idea that “violent gangsters suck”.
It is important to note that the article ends with an attempted disclaimer:

To be clear, police said vandalism on top of vandalism is still a crime. But his
group hopes that the ridicule will spur gang members to realize there is no
reason to be proud of their affiliation.

However, this may be the defining line between acceptable and unacceptable graffiti works.