“Can this have been quasi-intentional, a concerted effort to obliterate meaning by scrawling graffiti on one of the theater’s most profound texts?”
The above quote comes from a review of Young Jean Park’s play Lear! — a sort of reworking or reimagining of the Shakespeare play — and gives a window into the common usage of graffiti (as both a term and a concept).
Here, graffiti is an obscuring force, an ugliness of sorts that blocks out or obscures beauty or importance or whatever. Graffiti is scrawled, scribbled over something that was written, printed, bound, and later performed and experienced and theorized and beloved.
Graffiti obliterates meaning, distracts, takes away from, gouges. Graffiti is a destructive force, used to take away or to cover some quality of the surface on which it appears, into which it is scrawled.
Here, graffiti is deployed as negative, rather than a complex concept that consists of a wide variety of objects and activities. Indeed, there are forms of graffiti that would more properly be categorized as illuminating calligraphy, rather than obliterating scrawl. To limit graffiti to destructive acts ignores its potential to achieve beauty or serve as an enriching purpose. At the same time, to forget the destructive side of graffiti is to leave out an important facet of its character.
For example, the graffiti made by municipal workers on streets and sidewalks, which often indicate underground pipes and conduits. Such marks are important and useful, since they tell workers where and how deep to dig. They are graffiti, to be sure, but they are neither illicit nor destructive.
At this point, I believe it will be helpful to begin a glossary of sorts to name some specific forms or instances of graffiti and begin to make some sense of the graffiti milieu.
 Isherwood, Charles. “Blow, Winds! Deconstruct Thy Text.” New York Times, January 15, 2010. http://theater.nytimes.com/2010/01/15/theater/reviews/15lear.html?ref=arts (January 15, 2010).
 Additionally, as a word, ‘obliterate’ comes from the Italian obliterare, which has as its root littera, which refers to something written: to erase what is written. See http://dictionary.reference.com/browse/obliterate.