I highly recommend this documentary about the origins of Graffiti in NYC if you're interested in the subject.
It focuses on the subway a lot and has crazy shit in it like Cock putting up double fences with wolves in them.
Even if you want to eradicate it, you'd do best to actually try to understand it, and the people who do it, not simply dismiss them as criminals who should be locked up/shot.
https://www.youtube.com/watch?v=f9KxbaSU-Eo