One of the images on his web-site was of a spray-painted statue. I'd
call that defacing someone else's art. I enjoy the other work -- he's
very clever, although I don't always agree with his point of view,
but I guess that's the point.
Up here in Toronto, we've also had graffiti festivals, sanctioned by
the city. Artists paint designated walls and the backs of stores and
you can walk or drive down the alleys to see the work. There are also
projects done with kids at risk to paint the back alleys as well, but
these are more mural than grafitti. A far cry from tags (which are
all over the back of my husband's store-front ... the city insists he
clean it up or they will fine him!!! I call that chutzpah since he's a victim).