The following collection of street art is amazing and shows that art can be invited or not but will appear anyway: http://www.boredpanda.com/creative-street-art/
As Banksy and Loesje have shown (see Google Images) urban art can grow into quite valuable museum level art. City councils hate “grafitti art” but will live to regret that they order to remove it or paint it over, since it is often better and will be more valuable in the future than museum art. So: please do not fight these artists but give them room. Creativity makes life worth living in cities.