SAN FRANCISCO (AP) — San Francisco’s legal department is cracking down on graffiti artists who are spray-painting city sidewalks in a guerrilla ad campaign for Justin Bieber’s “Purpose” album. San Francisco City Attorney Dennis Herrara says he is pushing Bieber’s record label, Def Jam Records, to identify and punish those responsible for stenciling sidewalk ads that have not washed away after several rainstorms. Representatives for Bieber and for Def Jam did not return requests for comment.