Graffiti Artist Rules, OK

COPS CA’NT CLIMB. Big spray-painted letters, three storeys up. No useful handholds, no nearby drainpipes, no roof access.
PC Thorne wonders how they managed to get up there.
He fetches a ladder, a long one, pulls the aerosol can out his pocket, shakes it. He glances around; if someone catches him, he’ll be demoted or worse. He doesn’t care. He wants to teach these vandals a lesson.
He sprays, big letters, neat. And you can't write proper English. He underlines the can’t with a thick line of paint.
PC Thorne hates people who don’t know how to use apostrophes.