Property that is used by the public should be owned by the public. By that logic, graffiti artists as regular people who use public infrastructure in the area should as much right to paint on the wall as you do to paint the wall in your house. And you can paint over it if you hate it so much. But its all down to the community. It should not be a crime enforced by a ruling class who nominally owns the wall but doesn't use it, so is totally not actually inconvenienced by it being painted. You are part of the community. So is the artist. The person who owns the wall is mostly likely not. Stop pretending you're on the same side because you think you may also become a millionare by osmosis.